Local Test Environment with Docker Compose

[44203]

This PR includes a Dockerfile.dev and docker-compose.yml configured to give folks who want to hack on Tahoe-LAFS the option to stand up a minimal test environment by cloning the repository and simply running:

docker-compose up

The environment does the following:

• Create and run an introducer
• Create and run a storage server configured to connect to the introducer via docker's virtual network
• Create and run a client configured to connect to both of the above via docker's virtual network
• Publish ports for all three to the host
• Volume mount the repository on the host to the containers so changes are reflected

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[codecov-io]

Codecov Report

Merging #445 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
- Coverage   87.98%   87.98%   -0.01%     
==========================================
  Files         148      148              
  Lines       27557    27557              
  Branches     3953     3953              
==========================================
- Hits        24247    24245       -2     
- Misses       2602     2604       +2     
  Partials      708      708

Impacted Files	Coverage Δ
src/allmydata/immutable/upload.py	95.25% <0%> (-0.28%)	⬇️
src/allmydata/web/status.py	82.86% <0%> (+0.11%)	⬆️

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[warner]

Looks good! Landing (in rebased form) momentarily.