Add build hook for garage-base docker image

[gitanshu]

This image can then be used as the parent image for metaworld-ci docker image, instead of garage-headless as of now, which we discontinued in end of Jan 2021.

[codecov]

Codecov Report

Merging #2257 (cdf5d4b) into master (1e7ca02) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2257      +/-   ##
==========================================
+ Coverage   91.25%   91.28%   +0.02%     
==========================================
  Files         199      199              
  Lines       11761    10966     -795     
  Branches     1583     1387     -196     
==========================================
- Hits        10733    10010     -723     
+ Misses        758      694      -64     
+ Partials      270      262       -8     

Impacted Files	Coverage Δ
src/garage/torch/optimizers/optimizer_wrapper.py	93.75% <0.00%> (-6.25%)	⬇️
src/garage/sampler/env_update.py	82.92% <0.00%> (-3.62%)	⬇️
src/garage/torch/optimizers/differentiable_sgd.py	86.66% <0.00%> (-3.34%)	⬇️
src/garage/torch/policies/stochastic_policy.py	88.63% <0.00%> (-3.17%)	⬇️
src/garage/torch/algos/trpo.py	91.66% <0.00%> (-3.08%)	⬇️
src/garage/torch/algos/dqn.py	93.07% <0.00%> (-2.09%)	⬇️
src/garage/torch/_functions.py	92.42% <0.00%> (-1.92%)	⬇️
src/garage/torch/algos/sac.py	97.14% <0.00%> (-1.66%)	⬇️
src/garage/torch/modules/gaussian_mlp_module.py	92.13% <0.00%> (-1.52%)	⬇️
src/garage/sampler/multiprocessing_sampler.py	90.12% <0.00%> (-1.24%)	⬇️
... and 8 more

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