Document creation of /safe_data /safe_outputs /scratch

[mikej888]

Example Dockerfiles all do

RUN mkdir /safe_data /safe_outputs /scratch

or

RUN mkdir /safe_data /safe_outputs /scratch /test

docs/container-build-guide.md should document this requirement to allow a container to read/write data when run via the TRE Container Execution service

[rkm]

It might be worth discussing the wording of this, as it's really to highlight those directories are pre-created by the CES tools instead of being a hard requirement. Podman and docker (at least, maybe apptainer and k8s too) will automatically create any directories that are referenced if they don't already exist.