Base Locust Docker image on non-alpine python image

[heyman]

This PR makes Locust use the python:3.8 docker image instead of python:3.8-alpine as the base image.

The reason for this is that it'll make it possible to pip install ... most PyPI packages out of the box, which should be a very common thing to do in project specific docker images.

It comes with a ~900 MB cost in total image size, though I think the python:3.8 image should be a very common base image to already have pulled, in which case there will effectively be no higher disk usage, and I think it makes sense to prioritize developer convenience and happiness over disk usage.

This is what the description at the official docker python repo says about the python:3.8 image:

This is the defacto image. If you are unsure about what your needs are, you probably want to use this one. It is designed to be used both as a throw away container (mount your source code and start the container to start your app), as well as the base to build other images off of.

We could consider maintaining two branches of docker images (locustio/locust and locustio/locust-alpine), but I don't think it's worth the extra maintenance work.

We would have to bump the Locust version to 1.1 if we release this, since it might break end-users custom docker images (the fixes should be very easy though, and mostly consist of removing a bunch of lines no longer needed from their Dockerfiles).

[codecov]

Codecov Report

Merging #1435 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1435      +/-   ##
==========================================
+ Coverage   81.27%   81.31%   +0.04%     
==========================================
  Files          26       26              
  Lines        2344     2344              
  Branches      362      362              
==========================================
+ Hits         1905     1906       +1     
  Misses        348      348              
+ Partials       91       90       -1     

Impacted Files	Coverage Δ
locust/user/task.py	97.26% <0.00%> (+0.54%)	⬆️

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[cyberw]

Nice!

Can we drop the use of a builder image while we’re at it? I dont think it saves much (if anything) in image size now, and it simplifies things a little when users make derived images.

[cyberw]

Nice. +1

[heyman]

Great! I'll merge this, and then we'll bump the locust version to 1.1 with the next release.