docker setup: simplify and uniform to SWH path conventions

Major simplification pass on the docker documentation, Dockerfile, and scripts.

Main aim was to simplify the steps needed (as documented in docker.rst) and
reduce the amount of redundant information that need to be provided by the
user.

As this required a major overhaul, this revamp also includes:

• porting path conventions to SWH standards (e.g., everything is now under /srv/softwareheritage/graph, even in the container)

• sanitizing the main shell script