Container improvements

[NigelRook]

Provides two major benefits:

• Reduced image size to ~285Mb
• Now compatible with alpine's py3-* packages

Has the downside that this is potentially a breaking change for some people - the final image no longer contains rust/gcc, so if anybody is running any additional requirements that need those, they'd need to add them back through system_requirements.txt. This is mitigated somewhat by the fact that they might just be able to add an alpine package for it (I did this work for scikit-learn, which takes ~20 minutes to compile on my low-power i3 server, vs 20 seconds to install the alpine package + deps). That still requires a config change for people though, so I understand if you won't want to merge this. I'd be willing to add those packages back in to this PR if you'd prefer.

[acockburn]

I am worried about introducing breaking changes, especially since I don't use docker myself.

[acockburn]

If you can make the change to include alpine vs compile it I think that is a good compromise

[NigelRook]

OK updated

For clarity, I've:

• rebased onto latest dev
• added all alpine packages which were present in the current dev image
• bumped alpine to the latest stable release

Image isn't much smaller anymore, but alpine py3-* packages are still working.

[acockburn]

Thanks!

[acockburn]

I had to revert these commits because my docker pipeline started failing - not sure if this is the cause or not

[acockburn]

If/when I fix the build, I'd like to try again with this PR but it's merged and reverted and I am not sure there is a way I can re-merge it - could I ask you to create a fresh PR for this please?

Also, it seems your theory about memory and the size of the image was correct - I managed to get a build with one image, so now I am trying for 3. If that works I will call it good and drop support for linux/arm/v6. I have no idea if that is the right one to drop as I don't know who is even using AD ;) But I guess that the oldest non-intel image is probably the least used.

[NigelRook]

Here you go: #1655

Rebased and tested on my end

[acockburn]

Thanks!