docker: Only copy the control files in the container.

This avoids invalidating the cache when changing the rules file of the
packages, for example.

Automated via:

  $ git ls-files | grep Dockerfile | xargs sed -i 's,packaging/rules/debian-qt/\* /tmp/builddeps/debian/,packaging/rules/debian-qt/control /tmp/builddeps/debian/control,g'
  $ $ git ls-files | grep Dockerfile | xargs sed -i 's,packaging/rules/debian/\* /tmp/builddeps/debian/,packaging/rules/debian/control /tmp/builddeps/debian/control,g'

Change-Id: Ic33d7af4c022ed9de53df245c861a31d2786fc96