Sort out version reporting on Docker

[amontero]

Tahoe's official Docker image at ​https://hub.docker.com/r/tahoelafs/base/ reports version as 'unknown'. When testing beta, I thought it was a git issue or lack-of-tagging issue. Checking old issues I found that package version is tagged at build time from previous git commits, and thus can fail.

Since the initial Docker packaging from source, Tahoe-LAFS has become easier to install with time, for example via pip. My thoughts on this would be:

• Maintain the :devel tag, build directly from source

and either:

• Fix the git issue and keep building from source (I don't know enough to sort it out)
• Create a different Dockerfile which installs from pip for stable releases, which does carry version info. Looks easier and cleaner. Also builds on more tested pip packaging.

I can't also assess which one is more safe, so feedback welcome.

BTW, with the added new features, node creation is better, so my WIP on 1.11 to create different Docker images for different roles (client, introducer and storage) will be far easier. Thanks!