WEAV-3246: Dockerfile for weave device manager

[NLAVogel]

Also add init.py so when files are installed in dist-packages they
can be imported.

[codecov-io]

Codecov Report

Merging #249 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
- Coverage   54.22%   54.21%   -0.02%     
==========================================
  Files         332      332              
  Lines       56403    56403              
==========================================
- Hits        30586    30579       -7     
- Misses      25817    25824       +7

Impacted Files	Coverage Δ
third_party/lwip/repo/lwip/src/core/pbuf.c	35.2% <0%> (-1.6%)	⬇️
src/lwip/standalone/sys_arch.c	69.01% <0%> (-0.47%)	⬇️

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[jaylogue]

I'm wondering whether the Weave Device Manager will work correctly in all cases when run inside a Docker instance. In particular, will the Weave rendezvous protocol, which requires IPv6 multicast, work correctly between a Docker instance and a real device?

[jaylogue]

I think we should be using Makefile-Standalone whenever we build any of the desktop tools, rather than invoking ./configure with default args. The former employs project configuration files that configures OpenWeave specifically for desktop use.