Update the way of building gnmi target container in Makefile

Description

The containerize-target step in the Makefile is outdated and might cause issues in the future. Maybe we should move the related dockerfile away from the csbi subsection and update the build process within the file. That way we can use the automatically build container in all the clabs without any of the current problems.

Make sure to add --insecure flag to new gNMI-Target in clab.