Rename the dockerfile and add a dockerignore. This reduces the size of
the build-context greatly.
Renaming is a preparation for later adding more possible dockerfiles
(e.g. fedora based etc).
We choose the filename based `.dockerignore` files over a
`.dockerignore` in the root folder mainly to reduce clutter in the root
folder.
This addresses the problem of large build context raised on zulip by
@glyn.
Testing: Manually tested by building the image locally.
Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>
