Fix Dockerfile

[rikhuijzer]

The Dockerfile was broken. The error stated that the package libav-tools was unavailable.
To fix this I have fixed the Python version to python:3.7-stretch since python:3.7-buster does not contain libav-tools. See also the Debian package index.

[coveralls]

Coverage decreased (-0.02%) to 66.118% when pulling 7029d1f on rikhuijzer:patch-1 into 90e42c1 on Zulko:master.

[tburrows13]

I wonder if it works if you remove the installation of libav-tools? It seems to be a compatibility layer for ffmpeg, so perhaps we could instead get rid of it entirely? I think that it would be better to remove it than to pin it down to an older version of Debian?

@jonnekaunisto you had some experience with the Docker build, did you have any suggestions?

[jonnekaunisto]

The error that is thrown by the current Dockerfile is:

Package libav-tools is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source However the following packages replace it: ffmpeg

So I believe you can just replace it with ffmpeg instead.

[rikhuijzer]

Thanks @jonnekaunisto changing to ffmpeg is indeed a much better idea. I just did that and rebuild the Docker image. That works. However, I continued by trying to run some code which depends on ImageMagick and got unable to read font [...]. This is starting to become quite frustrating since I just spend my entire afternoon on a fruitless global MoviePy installation with ImageMagick on NixOS. So, I think this Dockerfile fix will be better than no fix, but it does not seem to deliver a version with a functioning ImageMagick attached.

[rikhuijzer]

This ImageMagick problem might be related to the fairly new comment at #855. I think it might be due to a missing font Courier. This font is used when calling the code from the docs.

[tburrows13]

In which case, I'm closing this in favour of #1165, which includes this change. I'm not sure about your ImageMagick problem... that program can be quite a pain at times. It is currently only used in TextClips and writing gifs. Gif writing already has alternatives that use different libraries (like pillow and ffmpeg directly), and it is likely that there will be a pillow alternative to TextClip in v2.0 (see #1145), which will heavily reduce the need for ImageMagick.

[rikhuijzer]

Sounds good. I now solved my ImageMagick problem on NixOS by switching my Python code to a font listed at convert -list font. I do still suspect that the default font for TextClip is Courier which causes problems in all my Docker builds (tried many Debian distributions).

[tburrows13]

fyi TextClip.list("font") also lists all fonts available to ImageMagick (all it is is a wrapper around convert -list font).