docker: use a "slim" base image

[chatziko]

This tiny PR changes the docker base image from python:3.8-buster to python:3.8-slim-buster. This most likely has no functional difference (tested only on a raspberry pi with gpiod), and reduces the uncompressed image from 855MB to just 144MB (pretty shocking, not sure why such a huge difference).

It also adds --no-cache-dir to pip, which saves a tiny bit more.

[BenjiU]

@chatziko: This changes breaks our delivery. Have you tested building with slim docker? Have you had to change something?

[chatziko]

@chatziko: This changes breaks our delivery. Have you tested building with slim docker? Have you had to change something?

I built (and activelly use in two different raspberry pis) an arm64 image using the exact committed Dockerfile, it worked fine. The exact build command:

docker buildx build --platform=linux/arm64 -t mqtt-io --load  .

I also built (but did not test) and amd64 image without problem.

What kind of problem did you get?

[BenjiU]

@chatziko: hmm, strange...
With your changes, we get https://github.com/flyte/mqtt-io/actions/runs/8055315759/job/22002238188#step:6:2738

#29 433.2 Failed to build msgpack
#29 433.2 ERROR: Could not build wheels for msgpack, which is required to install pyproject.toml-based projects

hmm, I need to look, what platform is causing the issue...

[chatziko]

arm/v7 is the problem, #352 likely fixes it.