tests/docker: avoid $SHELL invoke bash directly

tests/docker: avoid $SHELL invoke bash directly

On some images SHELL is pointing at a limited /bin/sh which doesn't
understand noprofile/norc. Given the run script is running bash just
invoke it directly.

This fixes:

  $ make docker-test-build@IMAGE DEBUG=1
  [...]
  + echo '  ./test-build'
  ./test-build
  + echo '* Hit Ctrl-D to continue, or type '\''exit 1'\'' to abort'
  * Hit Ctrl-D to continue, or type 'exit 1' to abort
  + echo
  + /bin/sh --noprofile --norc
  /bin/sh: 0: Illegal option --

Fixes: 2b0c4fa13f3
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>