Release v1.1.0

provisioning: add user field to container step

Add a `user` field on the container provisioning step that overrides
the user the container's command runs as. Plumbs through to
containerapi.WithUser, equivalent to `docker run --user`. Accepts a
numeric uid, uid:gid, or a name from the image's /etc/passwd; an empty
value keeps the image default.

Useful with execution-environment images (e.g. anything built by
ansible-builder) that default to a non-root uid: virter binds the SSH
key under /root/.ssh, which such users cannot read, so setting
`user = "0"` works as a workaround.