Add an assertion rather than an unused global for SWH_IMAGE_TAG

The SWH_IMAGE_TAG environment variable is used by Docker via
definition in mirror.yml used by docker stack deploy. The variable
itself is not used by the Python code and the eponymous global variable
was indeed unused.

Instead, it’s probably worth logging its value after a small assertion.
Not setting the variable would result in using latest image tag,
leading to weird results as Docker generally handles this tag pretty
inconsistently.