Update lf-docker-get-container-tag

Update lf-docker-get-container-tag

Allow shell script builder step to be customizable
in lf-docker-get-container-tag.
This will allow other teams like ONAP to call their
own script. Set to default to
"../shell/docker-get-container-tag.sh" which is what
teams were already using.

This feature is requested by ONAP's DCAE team to allow
them to handle tags differently via their own script.

Issue-ID: RELENG-2422
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>
Change-Id: I9afd52a0ad8916694db0e958e42eb80f98e4e28b