Flesh out imageTag in makefile

[nmdefries]

@dsweber2 noticed that "the variable imageTag in the makefile isn't defined; presumably its an environmental variable, but it isn't mentioned in the readme at all". I've set a default for imageTag in the Makefile and described it's use, BUT it turns out that the two make targets that use this variable aren't used anywhere else in our code. Building, tagging, and releasing is done explicitly in the ci.yaml workflow rather than using make.

These two targets could be useful if we need to manually build and release a docker image for production (e.g. something goes wrong with the automated release process while we're trying to release a hotfix). But we haven't needed this yet and it does make the code longer. I'm open to arguments either way.

Fixes #287

[krivard]

The targets are short so I say keep them in. The comments are a good reminder to double-check the yaml versions in case they lose sync.