Docker: pass USER and PWD environment bariables into container.
On each run, stack will test the stack root directory (~/.stack), and the
project and package work directories (.stack-work) for whether they are
owned by the current user and abort if they are not. This precaution can
be disabled with the --allow-different-user flag or allow-different-user
option in the global config (~/.stack/config.yaml).
Added stack clean --full option for full working dir cleanup.