packaging: replace dockerlib.sh with a Python script I want to do some more advanced things with Docker in upcoming commits. Trying to do that with shell scripts will be a bit too painful for my liking. Implementing things in Python will be vastly simpler in the long run. This commit essentially ports dockerlib.sh to a Python script. dockerdeb and dockerrpm have been ported to use the new hg-docker script. hg-docker requires Python 3. I've only tested on Python 3.5. Unlike the local packaging scripts which may need to run on old distros, the Docker packaging scripts don't have these constraints. So I think it is acceptable to require Python 3.5. As part of the transition, the Docker image tags changed slightly. I don't think that's a big deal: the Docker image names are effectively arbitrary and are a means to an end to achieve running commands in Docker containers. The code for resolving the Dockerfile content allows substituting values passed as arguments. This will be used in a subsequent commit. Differential Revision: https://phab.mercurial-scm.org/D3759

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