hub: improve docs for "buildContainer" RPC
hub: improve docs for "buildContainer" RPC

Reformat the description of the buildContainer RPC to Sphinx's doc
format, filling in the missing data types and options.

Signed-off-by: Ken Dreyer <940dddfbf8b0685439b7ecf27daa0ba58b359a2c@redhat.com>

release-engineering/koji-containerbuild,release-engineering/koji-containerbuild