offer helper for outdated apt-get

[maxheld83]

when you haven't recently run apt-get update, local_system_requirements() and friends can fail with something like:

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

I think this is because local_system_requirements() doesn't itself call apt-get update, and RSPM returns probably don't ever include it either. That makes sense, because apt-get update entails a global side effect which users might not want or anticipate.

On the other hand, it's easy to stub your toe on this especially inside cached docker images. And it's repetitive to have add a bunch of apt-get updates all over the Dockerfiles.

Proposed solutions (if I'm even right about this):

• offer a an additional update-apt argument?
• add to the documentation?

(Happy to draft a PR)