Some software (i.e. postgres) will be tagged with single version
numbers, like "12-20.04". These cases trigger an IndexError with the
current code. Instead, use slice notation to get "the first one or two
digits" and join(). This will prefer "M.N" style for multi-element
versions but allow "M" style when only one element is available.
Adjust HACKING_upstream.md to determine whether we're inside the "oci/" dir
Some images don't have an "oci/" directory where they put everything
related to the build system, so we have to adjust the "docker build"
command to adapt and only generate the "-f oci/Dockerfile.ubuntu ."
part when we're actually inside a directory called "oci".
HACKING_upstream.md is supposed to be used only by those images that
are built directly from the upstream source.
HACKING_docs.md is the documentation explaining how to hack on the
templates. It is common to both HACKING_deb.md and
HACKING_upstream.md, and is included by them.
