tests/docker: invoke the DEBUG shell with --noprofile/--norc

It's very confusing when things work in the debug shell because the
environment is different from what the test is running. Fix this by
ensuring we only have the inherited environment from the run shell.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-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
+$SHELL --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 && exit 1
+    $SHELL --noprofile --norc && exit 1
 else
     exit 1
 fi