I’ve stopped using docker-compose

docker-compose is a very handy tool when you want to run multi container installations. Using a very simple YAML description, you can develop stuff locally and then push upstream whenever you feel something needs to enter the CI/CD cycle.

Sometimes, I’ve even used it on production, when I wanted to coordinate certain containers on a single VM. But lately I’ve stopped doing so. The reason is simple:

All that we do is ultimately going to be deployed in a Kubernetes cluster somewhere.

Given the above, there’s no need to maintain two sets of YAMLs, one for the docker-compose.yaml and one for the Kubernetes / helm manifests. Just go with Kubernetes from the beginning. Run a cluster on your local machine (Docker Desktop, microk8s, or other) and continue from there. Otherwise you risk running into the variation of works on my machine that is phrased like but it works with docker-compose. Well, there’s no docker-compose in production, why should there be on your machine? Plus you’ll get a sense of how things look like in production.

If you’re so much used to working with docker-compose, you can start a very crude transition by assuming that you have a single deployment and every container that you were to deploy is a side-car container to a single Pod. Afterall, just like a Pod, any docker-compose execution cannot escape a single machine (yes I know about Swarm). Then you can break it down to different deployments per container you want to run.

The above occured to me when I was trying to deploy some software locally, before deploying on Kubernetes, and tried to follow the vendor instructions for docker-compose. They failed and I lost quite some time trying to fix the provided YAML, and it dawned me: I do not need it. I need to test in Kubernetes anyway.

So there, stop using docker-compose when you can. Everyone will be happier.

podman completion zsh

I removed Docker Desktop from my machine in favor of podman. Nothing wrong with Docker Desktop, it still rocks, but I own a copy of Podman in Action, so I need to go through with it. As anyone with zsh who follows fashion, I am using oh my zsh!, so I wanted to enable podman’s command line completion to it. It turns out, others had the same issue some years back and the solution is:

mkdir -p $ZSH_CUSTOM/plugins/podman/
podman completion zsh -f $ZSH_CUSTOM/plugins/podman/_podman

In the end, don’t forget to enable the plugin in your ~/.zshrc In my case for example, I enable the following plugins:

plugins=(git fzf direnv brew tmux golang kubectl helm podman)

My .wslconfig for Docker Desktop

I was running some tests within my Docker Desktop and it started not responding. I restarted it and it failed to start. So I thought, OK this is a resource problem. I opened the panel to configure the resources. But since it is integrated with WSL2, it said it needed a .wslconfig file. I’d never configured that before and where would I place it? It resides in %PROFILE%\.wslconfig thankfully and my quick googling around revealed a set of options that made it work:


removing all containers via ssh

Assuming one wants to unconditionally remove all containers in a host, they would run:

$ docker rm -f $(docker ps -a -q)

Suppose now that you want to execute this in a remote host via ssh. The following won’t work:

$ ssh remote_user@remote_host "docker rm -f $(docker ps -a -q)"

because the $(docker ps -a -q) part is expanded locally on your machine and you ssh remotely after the expansion. So at best it would seek to remove your local containers over there :)

What would work though? xargs to the rescue:

$ ssh remote_user@remote_host "docker ps -a -q|xargs -n 1 docker rm -f "

Of course you can fine-tune further if you wish.

RUN –mount=type=ssh is not always easy

Let’s take a very barebones Jenkinsfile and use it to build a docker image that clones something from GitHub (and possibly does other stuff next):

pipeline {
  agent any

  environment {

  stages {
    stage('200ok') {
      steps {
        sshagent(["readonly-ssh-key-here"]) {
          script {
            sh 'docker build --ssh default -t adamo/200ok .'

We are using the SSH Agent Plugin in order to allow a clone that happens in the Dockerfile:

# syntax=docker/dockerfile:experimental
FROM bitnami/git
RUN mkdir /root/.ssh && ssh-keyscan github.com >> /root/.ssh/known_hosts
RUN --mount=type=ssh git clone git@github.com:a-yiorgos/200ok.git

This builds fine. But what if you need this to be some "rootless" container?

# syntax=docker/dockerfile:experimental
FROM bitnami/git
USER bitnami
WORKDIR /home/bitnami
RUN mkdir /home/bitnami/.ssh && ssh-keyscan github.com >> /home/bitnami/.ssh/known_hosts
RUN --mount=type=ssh git clone git@github.com:a-yiorgos/200ok.git

This will fail with something like:

#14 [7/7] RUN --mount=type=ssh git clone git@github.com:a-yiorgos/200ok.git
#14       digest: sha256:fb15ac6ca5703d056c7f9bf7dd61bf7ff70b32dea87acbb011e91152b4c78ad4
#14         name: "[7/7] RUN --mount=type=ssh git clone git@github.com:a-yiorgos/200ok.git"
#14      started: 2021-12-17 12:00:22.859388318 +0000 UTC
#14 0.572 fatal: destination path '200ok' already exists and is not an empty directory.
#14    completed: 2021-12-17 12:00:23.508950696 +0000 UTC
#14     duration: 649.562378ms
#14        error: "executor failed running [/bin/sh -c git clone git@github.com:a-yiorgos/200ok.git]: exit code: 128"

rpc error: code = Unknown desc = executor failed running [/bin/sh -c git clone git@github.com:a-yiorgos/200ok.git]: exit code: 128

Why is that? Is not the SSH agent forwarding working? Well, kind of. Let’s add a couple of commands in the Dockerfile to see what might be the issue:

# syntax=docker/dockerfile:experimental
FROM bitnami/git
USER bitnami
WORKDIR /home/bitnami
RUN mkdir /home/bitnami/.ssh && ssh-keyscan github.com >> /home/bitnami/.ssh/known_hosts
RUN --mount=type=ssh env
RUN --mount=type=ssh ls -l ${SSH_AUTH_SOCK}
RUN --mount=type=ssh git clone git@github.com:a-yiorgos/200ok.git

Then the build output gives us:

#13 [6/7] RUN --mount=type=ssh ls -l ${SSH_AUTH_SOCK}
#13       digest: sha256:ce8fcd7187eb813c16d84c13f8d318d21ac90945415b647aef9c753d0112a8a7
#13         name: "[6/7] RUN --mount=type=ssh ls -l ${SSH_AUTH_SOCK}"
#13      started: 2021-12-17 12:00:22.460172872 +0000 UTC
#13 0.320 srw------- 1 root root 0 Dec 17 12:00 /run/buildkit/ssh_agent.0
#13    completed: 2021-12-17 12:00:22.856049431 +0000 UTC
#13     duration: 395.876559ms

and subsequently fails to clone. This happens because the socket file /run/buildkit/ssh_agent.0 for the SSH agent forwarding is not accessible by user bitnami and thus no ssh identity is available to it.

I do not know whether it is possible to make use of RUN --mount=type=ssh in combination with USER where the user is not root. Please leave a comment if you know whether/how this can be accomplished.

Using systemd-nspawn to run an older Ubuntu version and Docker images

Work for this post was partly triggered by something I saw at work and partly because of a lightning talk I saw on Monday. There was a remark by a friend that even though we love and want to run latest and greatest supported versions, we cannot always upgrade in time. And as such, sometimes we end-up with unsupported OS versions for a significant time. Not unlike the law of welded systems.

But it got me thinking. Assuming, for example, that you have a number of Xenial systems that due to library dependencies you cannot upgrade, is there a middle ground that might be acceptable for a while until you push forward? Using a Xenial docker container and treating it as a lightweight VM somehow, could be a solution. But what if you need to run docker containers too? Maybe you need something more elaborate.

I fired up a VM running Ubuntu Focal and decided to figure out how to run a systemd-nspawn Xenial container.

Create the machine:

# debootstrap --arch=amd64 xenial /var/lib/machines/xenial1

Start the machine

# systemd-nspawn -D /var/lib/machines/xenial1
root@xenial1 # rm /etc/securetty
root@xenial1 # passwd root
root@xenial1 # apt-get update
root@xenial1 # apt-get install dbus resolvconf
root@xenial1 # systemctl enable systemd-resolved
root@xenial1 # cat > /etc/resolvconf/resolv.conf.d/base
options edns0 trust-ad
search home
root@xenial1 #

Yes, I know that it is best to fix securetty instead of removing it, but this is a PoC on my VM. You can now proceed to configure systemd to run the container:

# /etc/systemd/system/xenial1.service

Description=Xenial1 Container

ExecStart=/usr/bin/systemd-nspawn --machine=xenial1 --directory=/var/lib/machines/xenial1/ --bind /var/run/docker.sock:/var/run/docker.sock --bind /mnt2:/mnt2 -b 


In your host system you can now:

root@focal # systemctl daemon-reload
root@focal # systemctl start xenial1

You can of course log into the machine with machinectl login xenial1. Notice above that the nspawn container mounts the docker socket, since we assume that you have docker installed in the Focal host and that you want to "run" docker containers from within the Xenial machine.

We only need to install the docker client in the Xenial container and as such:

root@xenial1 # cat > /etc/apt/sources.list.d/docker.list
deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu xenial stable
root@xenial1 # apt-get update
root@xenial1 # apt-get install docker-ce-cli=5:18.09.7~3-0~ubuntu-xenial

Now you’re all set. The docker client inside Xenial, starts whatever container you want in Focal and does whatever you like.

Suppose that you want a non-root user like ubuntu in the Xenial machine to be able to run docker commands; what do you do? You add the /etc/group line for the docker group from Focal in Xenial and in Xenial you simply useradd -aG docker ubuntu.

ubuntu@xenial1 > docker run -d -p 8080:8080 bitnami/nginx
ubuntu@focal > curl
If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.


So is any string proper for a docker image tag?

There was a failure for a build in our system that looked like this:

docker build -t yiorgos/my-cool-application:service/1.1.2 .

invalid argument "yiorgos/my-cool-application:service/1.1.2" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.

Sometimes you get this error when you forget a space between the dot (the current directory where your Dockerfile usually lives and the tag). But this was not the case. Docker actually did not like the tag for the image.

My hunch was that it does not like slashes inside the actual tag part (right of the :).

And indeed by checking out the source code of podman we that this is indeed the case:

//	tag                             := /[\w][\w.-]{0,127}/

reference.go contains the full specification of a tag for anyone interested.

sometimes you need to change your docker network

I was working locally with some containers running over docker-compose. Everything was OK, except I could not access a specific service within our network. Here is the issue: docker by default assigns IP addresses to containers from the pool and docker-compose from It just so happened that what I needed to access lived on space. So what to do to overcome the nuisance? Obviously you cannot renumber a whole network for some temporary IP overlapping. Let’s abuse reserved IP space instead. Here is the relevant part of my daemon.json now:

  "default-address-pools": [
    { "base": "", "size": 28 },
    { "base": "", "size": 28 },
    { "base": "", "size": 28 }

According to RFC5737 are reserved for documentation purposes. I’d say local work is close enough to documentation to warrant the abuse, since we also adhere to its operational implications. Plus I wager that most of the people while they always remember classic RFC1918 addresses, seldom take into account TEST-NET-1 and friends.

Sometimes you need JDK8 in your docker image …

… and depending the base image this may not be available, even from the ppa repositories. Moreover, you cannot download it directly from Oracle without a username / password anymore, so this may break automation, if you do not host your own repositories. Have no fear, sdkman can come to the rescue and fetch for you a supported JDK8 version. You need to pay some extra care with JAVA_HOME and PATH though:

FROM python:3.6-slim
RUN apt-get update && apt-get upgrade -y
RUN apt-get install -y curl zip unzip
RUN curl -fsSL https://get.sdkman.io -o get-sdkman.sh
RUN sh ./get-sdkman.sh
RUN bash -c "source /root/.sdkman/bin/sdkman-init.sh && sdk install java 8.0.212-amzn"
ENV JAVA_HOME=/root/.sdkman/candidates/java/current
CMD bash

Since curl | sudo bash is a topic of contention, do your homework and decide whether you’re going to use it as displayed in the Dockerfile above, or employ some extra verification mechanism of your choice.

minikube instead of docker-machine

docker-machine is a cool project that allows you to work with different docker environments, especially when your machine or OS (like Windows 10 Home) does not allow for native docker. My primary use case for docker-machine was spinning up a VirtualBox host and using it to run the docker service.

It seems that docker-machine is not in so much active development any more and I decided to switch to another project for my particular use case: Windows 10 Home with VirtualBox. One solution is minikube:

minikube start
minikube docker-env | Invoke-Expression # Windows PowerShell
eval $(minikube docker-env) # bash shell
docker pull python:3
docker images

and you’re set to go.