docs(docker): Add ID mapping to docker commands

[gumbo2k]

Without this, the workspace files are created with uid/gid 0 and thus belong to "root". They can then not be edited by the user who invoked the docker command in the first place.

[gumbo2k]

I've just found the hidden feature to USE_SYSTEM_CA_CERTS .
When using the uid:gid mapping as I proposed in this PR, then the USE_SYSTEM_CA_CERTS feature breaks.
I think I'll have to incorporate the uid:gid mapping by passing this as an ENV variable to the container, and changing uid:gid at the last moment (when running exec "$@" in /__cacert_entrypoint.sh )