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

packaging: consistently create build user in Dockerfiles Previously, dockerlib.sh appended some commands to create a "build" user in each Docker image. The resulting Docker images could be inconsistent depending on the execution environment and base image. With this change, we explicitly create our custom user and group as the first action in each Dockerfile. The user always has user:group 1000:1000 and all built images are consistent. We also create a home directory for the user under /build. This directory is currently ignored. As part of this, we stop setting the DBUILDUSER variable in dockerlib.sh and instead set it in the respective scripts that call it. This is in preparation for further refactoring of dockerlib.sh. Differential Revision: https://phab.mercurial-scm.org/D3758