Remove non-public images from the git-sha-based target determination

[VJalili]

This PR excludes the non-public docker images (melt specifically`) from the git-sha-based target images determination.

These images can still be built if listed explicitly via the --targets argument.

[cwhelan]

I think it's right to exclude these from automatic builds based on dependencies, since you need to provide the non-public melt binary file. To avoid confusion I think it might be good to print a quick warning about this (eg something like "Warning: Skipping build of non-public melt docker even though dependencies have changed; please update these images manually") -- what do you think?

[VJalili]

Thank you for reviewing this, @cwhelan! I agree printing a warning message would be helpful. I updated accordingly.

[cwhelan]

LGTM now.

[VJalili]

I updated the script to the non-public images (melt specifically) that are also skipped from the dependency tree. Also, I added print statements that indicate the images are omitted.

One way to test this would be the following command, and you can check if the non-pub images are to be built by considering the shown print statement.

$ python scripts/docker/build_docker.py --targets sv-base-virtual-env --disable-git-protect
...
Building the following targets in order:
['sv-base-virtual-env', 'cnmops-virtual-env', 'sv-pipeline-virtual-env', 'sv-base', 'cnmops', 'sv-pipeline']

Note that melt is not included in the above list.

[cwhelan]

Looks good to me.

[VJalili]

Thanks, @cwhelan!