Add support for running CompilerGym environments from docker containers

[ChrisCummins]

This adds a chriscummins/compiler_gym docker image that consists of a pre-configured CompilerGym install in a Python 3.8 Linux environment. Changes required to implement this:

• Made Benchmark.from_file() compatible with distributed services.
• Fixed compatibility of the --service and --local_service_bin flags.
• Fixed an unrelated bug when changing reward space midway through an episode.
• Added a --run_on_port flag to compiler_gym.bin.service.

[codecov-commenter]

Codecov Report

Merging #424 (ee42fd5) into development (03f0a3b) will decrease coverage by 0.01%.
The diff coverage is 67.64%.

@@               Coverage Diff               @@
##           development     #424      +/-   ##
===============================================
- Coverage        87.29%   87.28%   -0.02%     
===============================================
  Files              100      100              
  Lines             5746     5763      +17     
===============================================
+ Hits              5016     5030      +14     
- Misses             730      733       +3     

Impacted Files	Coverage Δ
compiler_gym/bin/random_search.py	0.00% <0.00%> (ø)
compiler_gym/service/connection.py	77.58% <ø> (+1.03%)	⬆️
compiler_gym/bin/service.py	76.27% <45.45%> (-7.41%)	⬇️
compiler_gym/util/flags/env_from_flags.py	89.58% <76.92%> (+4.16%)	⬆️
compiler_gym/datasets/benchmark.py	99.00% <100.00%> (+0.01%)	⬆️
compiler_gym/envs/compiler_env.py	88.27% <100.00%> (-0.30%)	⬇️
compiler_gym/spaces/reward.py	72.54% <0.00%> (-1.97%)	⬇️
compiler_gym/envs/llvm/llvm_env.py	85.24% <0.00%> (-0.82%)	⬇️
compiler_gym/envs/gcc/service/gcc_service.py	97.37% <0.00%> (ø)
... and 1 more

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