Simplify Docker container

[AdamGleave]

• Split Dockerfile into stages.
• CI now uses base stage, which excludes venv, so as not to duplicate venv given CircleCI caching.
• Downgrade from MuJoCo 2.0 to 1.5 (needed for recent Gym versions).

[codecov[bot]]

Codecov Report

Merging #5 into master will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   71.31%   71.33%   +0.01%     
==========================================
  Files          39       39              
  Lines        2367     2365       -2     
==========================================
- Hits         1688     1687       -1     
+ Misses        679      678       -1

Impacted Files	Coverage Δ
tests/test_policies.py	100% <ø> (+2.12%)	:arrow_up:

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