Move build image code to shared parent class

[nweires]

Pull Request Description

Move build_image() to docker_base.py, so it can be shared by both aws.py and gcp.py.

The only logic change is renaming the . aws_docker_image/ directory to .cloud_docker_image. As far as I can tell, nothing else depends on this directory, but let me know if there's a reason not to change it.

Checklist

Not all may apply

• [x] Code changes (must work)
• [x] Tests exercising your feature/bug fix (check coverage report on Checks -> BuildStockBatch Tests -> Artifacts)
• [x] Coverage has increased or at least not decreased. Update minimum_coverage in .github/workflows/coverage.yml as necessary.
• [x] All other unit and integration tests passing
• [ ] Update validation for project config yaml file changes
• [ ] Update existing documentation
• [ ] Run a small batch run on Kestrel/Eagle to make sure it all works if you made changes that will affect Kestrel/Eagle
• [ ] Add to the changelog_dev.rst file and propose migration text in the pull request

[github-actions[bot]]

File	Coverage
All files	87%	:white_check_mark:
base.py	92%	:white_check_mark:
exc.py	57%	:white_check_mark:
hpc.py	78%	:white_check_mark:
local.py	70%	:white_check_mark:
postprocessing.py	85%	:white_check_mark:
utils.py	92%	:white_check_mark:
cloud/docker_base.py	89%	:white_check_mark:
sampler/base.py	79%	:white_check_mark:
sampler/downselect.py	33%	:white_check_mark:
sampler/precomputed.py	93%	:white_check_mark:
sampler/residential_quota.py	61%	:white_check_mark:
test/shared_testing_stuff.py	85%	:white_check_mark:
test/test_docker.py	33%	:white_check_mark:
test/test_local.py	97%	:white_check_mark:
test/test_validation.py	97%	:white_check_mark:
workflow_generator/base.py	90%	:white_check_mark:
workflow_generator/commercial.py	53%	:white_check_mark:
workflow_generator/residential_hpxml.py	86%	:white_check_mark:

Minimum allowed coverage is 33%

Generated by :monkey: cobertura-action against dbcdfec200ea726b4eca769603b38f40e9825585