Mount the source code directory as a volume in the hotpotato-test Docker Compose service

Closes #101 (closed)

Also does the following:

• Adds the theme service as a dependency of hotpotato-test, to build the theme resources in the source directory
• Changes the Add New User robot test to check the ID of the element used to store the password, rather than the tag (found this while debugging, doesn't fix anything but more ideal way of checking)