tests/lcitool: append user setting stanza to dockerfiles

tests/lcitool: append user setting stanza to dockerfiles

For the cross-compilation use-case it is important to add the host
user to the dockerfile so we can map them to the docker environment
when cross-building files.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230228190653.1602033-19-alex.bennee@linaro.org>