docker: manifest push: Ignore missing images

Certain containers might lack one of the images that should be part
of the multiarch tag, so create the manifest without them and do not
bail in such scenarios.

Change-Id: Ia1094d1dfb1c998d7eda5e68c751d75ab3966c5b
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>