validation: Move docker build job to aarch64_build

validation: Move docker build job to aarch64_build

Previously, validation docker build jobs used to run on 'aarch64_dev'
labeled slaves, which are also used by IEC installers (compass, fuel)
that interfere with the machine's Docker service (usually by restarting
the Docker service when the install/deploy is started; which
unfortunately overlaps quite frequently with the validation docker build
job). Of course, this is also possible due to the 'aarch64_dev' nodes
having more than 2 executors (currently 3).

To avoid this, we could add build-blockers to both IEC install jobs and
validation docker build jobs, but that would create unnecessary cross
dependencies between IEC and validation projects.
Instead, move the validation Docker build to nodes servicing the label
'aarch64_build', which only have 2 executors and currently only run TA
RPM/ISO build jobs that do not interfere in a destructive way with the
Docker service on the machine.

JIRA: VAL-101

Change-Id: Iebc57a6a710b96ee6dcb04ea5e98243af3b44fa8
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>