Currently the docker runtime always sets $HOME to /. We could offer conveniences to detect a more relevant value based on the current user.

One way is to parse /etc/passwd and look for the home directory corresponding to the current user. We already parse it when a user is configured with run -u, so why not?

Another way is to extend the WORKDIR build directive (which specifies which directory to execute the process in), and have it set $HOME to that value. We could rename that directive to "HOME" to make that more obvious.