The user, working directory and environment commands run with.
Mirrors docker run -u / -w / -e: user and work_dir replace the layer
below, env_vars merge into it key by key. It applies at three points, each
layered over the one before -- the snapshot, the sandbox, and a single
command.
Environment variables, merged over the layer below.
Account to run as: name, uid, name:group or uid:gid. Defaults to
the Docker image's USER.
Absolute working directory. Defaults to the image's WORKDIR. A relative
path is rejected by the server.
