Add reasonable defaults for packaging environment.

Author: guzman-raphael

Dockerfile

@@ -1,6 +1,6 @@
-ARG IMAGE
-ARG PY_VER
-ARG DISTRO
+ARG IMAGE=djbase
+ARG PY_VER=3.9
+ARG DISTRO=debian
 FROM datajoint/${IMAGE}:py${PY_VER}-${DISTRO}
 COPY --chown=anaconda:anaconda ./setup.py ./datajoint.pub ./requirements.txt /main/
 COPY --chown=anaconda:anaconda ./datajoint /main/datajoint