[docker] Fix wrong dependency in build target

Previously, we relied on the '$<' Makefile automatic variable to
determine the directory where a certain build target should run.
Although this worked for a single container (as '$<' expands to
the *first* dependency of the make target), it causes issues when
multiple subdirectories are present, since all but the first
target will have '$<' pointing to the same first subdirectory.

Instead, switch to using '$@' which always expands to the current
make target and perform some simple string manipulation to extract
the proper target directory where the build should occur.

JIRA: VAL-27

Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
Change-Id: Id8448c6feffb04dc14c8cd1c3d362f1d2934fd24