Allowing opening shell as root / sudo user

[joehoyle]

To allow people to debug / install packages, they need a root permissions level user in the shell command. Passing -u root to the command would suffice.

Acceptance criteria:

• Add an option flag to the shell command called --root, optional, alias as -r
• When flag is set add -u root to the docker exec call
• Update the commands documentation with this argument
• Add a note to the command docs on why you would use the root user:
  • To test modifications to the PHP container eg. installing PHP libraries or other packages
  • To carry out any other tasks that require sudo privileges such as making directories outside of the app directory