Expand Docker utilities

[mattBrzezinski]

• Add Docker.tag (https://gitlab.invenia.ca/invenia/AWSClusterManagers.jl/merge_requests/70/diffs#note_81740)
• Support --build-arg in Docker.build (https://gitlab.invenia.ca/invenia/AWSClusterManagers.jl/merge_requests/70/diffs#note_81740)