docker_workspace: support dockerignore to filter files from the workspace

[d4l3k]

This adds a filter when building the docker workspace context to filter files specified in the .dockerignore file. This reduces the size of the patch when operating in a larger repo.

Reference:
https://docs.docker.com/engine/reference/builder/#dockerignore-file

Next steps from this would be to extend this logic to be workspace agnostic so we could have a .torchxignore file but not sure if that makes sense or is strictly necessary yet.

Test plan:

$  torchx run --scheduler aws_batch --wait --log dist.ddp --script foo.py  -j 1x1
$ pytest torchx/workspace/

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[codecov]

Codecov Report

Merging #401 (6128afe) into main (8a7a95a) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
+ Coverage   94.50%   94.53%   +0.03%     
==========================================
  Files          66       66              
  Lines        3638     3663      +25     
==========================================
+ Hits         3438     3463      +25     
  Misses        200      200              

Impacted Files	Coverage Δ
torchx/workspace/docker_workspace.py	95.19% <100.00%> (+1.52%)	⬆️

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