docker: Support running in Rootless Docker mode

We require support for Rootless Docker (a different method of running
docker which does not require any root privileges). For rootless docker,
all bind mounts are done in root mode which gives access errors when
attempting to run docker with the user being the non-root user.

The recommended method to get around this issue is to run docker without
passing the --user flag and therefore be the root user in the container.
Note that rootless docker will ensure that all created files have user
permissions instead of root.

We need to override the entrypoint in this case, as the default
entrypoint checks the user and prevents running as root.

Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>