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: 2b0c4fa13f
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>

tests/docker/run

@@ -62,7 +62,7 @@ echo "* Prepared to run command:"
 echo "  $CMD"
 echo "* Hit Ctrl-D to continue, or type 'exit 1' to abort"
 echo
-$SHELL --noprofile --norc
+env bash --noprofile --norc
 
 if "$CMD"; then
     exit 0
@@ -72,7 +72,7 @@ elif test -n "$DEBUG"; then
     echo "* Hit Ctrl-D to exit"
     echo
     # Force error after shell exits
-    $SHELL --noprofile --norc && exit 1
+    env bash --noprofile --norc && exit 1
 else
     exit 1
 fi