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docker contexts are great, would recommend putting them in your prompt though (via starship or something else)… here’s why

I like to manage my containers remotely - I have a nice development setup on my desktop and I try to keep my server as bare-bones as possible. For a while I’ve been using ansible which makes it easy to manage configuration etc on other machines. But I recently learned about docker contexts and I’m planning to scale down my homelab management to just docker-compose stacks rather than a bunch of super complicated ansible playbooks

So, setting up a context is easy - it’s basically an ssh connection to another machine!

docker context create koober --docker "host=ssh://nic@koober"

koober is one of my dev machines and my ~/.ssh/config is setup such that I can ssh nic@koober, this makes the context work really seamlessly.

So there’s the default context (the machine you’re on) and now I have koober

To use it you run docker context use koober

And then to check we can ls the contexts

docker context ls
NAME       DESCRIPTION                               DOCKER ENDPOINT               ERROR
default    Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   
koober *                                             ssh://nic@koober

Notice the * - that indicates that’s our current context.

Trouble #

Now here’s where things get hairy… you’ve gotta be super-aware of what context you’re using. I have an indicator in my starship prompt that shows the current context, but since I’m new to using them I kind of didn’t notice it until I ran into this issue…

I’m working on a python application in docker but was not able to execute the entrypoint even though I KNEW the file was there… let’s take a look

Example #

Here’s a minimal hello world applycation in docker to illustrate the issue

# main.py
print("hello world")

# Use an official Python runtime as a parent image
FROM python:3.11-slim

# Set the working directory in the container
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Run the main.py script as the container's main process
CMD ["python", "main.py"]

services:
  hello-world:
    build: .
    volumes:
      - .:/app

Notice volume mounting in my project directory . to /app as a common practice to develop inside the container

Here’s where I started to question my sanity…

✗ docker compose up                                            
[+] Running 1/0
 ✔ Container docker-context-example-hello-world-1  Created                                                                                                                                                                                                                                                              0.0s 
Attaching to hello-world-1
hello-world-1  | python: can't open file '/app/main.py': [Errno 2] No such file or directory
hello-world-1 exited with code 2

python can't open file? hmm… Let’s take a look at the image

First let’s make sure we get the image name right

✗ docker container ls -a              
CONTAINER ID   IMAGE                                           COMMAND                  CREATED          STATUS                       PORTS                                                                                  NAMES
3d7285fc39e2   docker-context-example-hello-world              "python main.py"         10 minutes ago   Exited (2) 44 seconds ago                                                                                           docker-context-example-hello-world-1

Now we can docker run --rm -it --entrypoint /bin/bash --name debug docker-context-example-hello-world

❯ docker run --rm -it --entrypoint /bin/bash --name debug docker-context-example-hello-world  
root@ee46d0e22de8:/app# python main.py
Hello, World!
root@ee46d0e22de8:/app# 

WHAT THE HECK??

What happened… #

What happened turns out to be pretty simple once we realize I’m using contexts…

koboer is a remote context, the docker run and docker compose up commands are interacting with the docker socket on that machine.

So if I compose up the stack notice that there’s a volume bind mount in there - well those do not work with contexts (or at least I’m not aware of hose to make it work) and so the /app directory was getting blown away essentially with an empty overlay…

But when running with just docker run with no volume mount, the code was copied in during the build and is right where we expect it…

Let’s prove it #

❯ docker run --rm -it --entrypoint /bin/bash --name debug -v .:/app docker-context-example-hello-world
root@903f591c0384:/app# python main.py
python: can't open file '/app/main.py': [Errno 2] No such file or directory
root@903f591c0384:/app# 

Adding a `-v .:/app` to match the compose file, we get the same error...

If we switch to the default context we are back up and running as expected

✗ docker context use default
default
Current context is now "default"

nic in /tmp/docker-context-example  via   v3.13.0  (dev) 󰒄 󱔎 NO PYTHON ENVIORNMENT SET 
❯ docker compose up         
[+] Running 1/0
 ✔ Container docker-context-example-hello-world-1  Created                                                                                                                                                                                                                                                              0.0s 
Attaching to hello-world-1
hello-world-1  | Hello, World!
hello-world-1 exited with code 0

nic in /tmp/docker-context-example  via   v3.13.0  (dev) 󰒄 󱔎 NO PYTHON ENVIORNMENT SET 
❯ docker run --rm -it --entrypoint /bin/bash --name debug -v .:/app docker-context-example-hello-world
root@4045b6aa8883:/app# python main.py
Hello, World!
root@4045b6aa8883:/app# 

Successful runs on both accounts with the volume mount

TLDR #

Context is king