Update lf-docker-jobs to enforce macro vars

Enforce the variables from "lf-docker-get-container-tag",
"lf-docker-build" and "lf-docker-push" to be carried over
when calling the macros. This avoids the issue of loosing
the value of the variables.
Update the test file to reflect the example better.

Change-Id: I5087b6fbd064d145fab4be8dad72ff196fdfa213
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>