ci: Edit Dockerfile.petsc and add docker-petsc.yml

[ZoeLeibowitz]

While developing on the petsc branch, this PR adds docker-petsc.yml to build and push Dockerfile.petsc on pushes to the petsc branch. This PR also updates Dockerfile.petsc so that it uses devitocodes/devito:cpu-dev as its base. Marked as a draft until PR #2778 is reviewed/merged.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 75.98%. Comparing base (9451510) to head (17cc296).

Files with missing lines	Patch %	Lines
devito/petsc/config.py	57.14%	3 Missing :warning:
devito/petsc/solver_parameters.py	80.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##            petsc    #2779      +/-   ##
==========================================
- Coverage   75.98%   75.98%   -0.01%     
==========================================
  Files         273      273              
  Lines       53844    53850       +6     
  Branches     4551     4551              
==========================================
+ Hits        40913    40917       +4     
- Misses      12106    12108       +2     
  Partials      825      825              

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