umask of containers created from python3 Dockerfile does not match Diamond default

For the case where:

- You create a containerised service
- Which mounts the Diamond filesystem
- And create files which are intended to be temporary, interleved with existing files

The current umask means those temporary files cannot be deleted by the user that created the directory that stores those files.
E.g. __pycache__ files made during an editable install of a library loaded from filesystem for blueapi, checked out by a member of dls_dasc

For blueapi (where we meet these requirements) we are [now setting the umask](https://github.com/DiamondLightSource/blueapi/pull/480/files): is anyone else going to meet those requirements and if so should this be configurable as part of the template, perhaps propagating into the Dockerfile?