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.