ability to set custom tag, somewhat addresses #213

[parisholley]

I have a repo that has all my docker images, prefer not to have to push them to a repo in order to reference images by name. I use fig.yml to build all images locally, which can than be reused through DockerFiles, etc.

[dnephin]

Some things you'll probably be asked for:

• the build is failing because of a flake8 check, you'll need to fix the whitespace
• commit message should include a signoff, see Contributing
• docs should probably be updated to include this

[dnephin]

I think another issue may be that when you change the tag name it may have trouble finding the containers for this image, because it assumes the naming convention is always project_service_index.

I think leaving that assumption as-is should be fine. Instead I think we can docker tag for each tag name when the image is built.

[dnephin]

#457 should add full support

[dnephin]

#1066 will be in the next release, which will let us do #652

Closing this PR in favor of the explicit naming proposed in those tickets