Allow specifing docker external port (#1490)
Allow specifing docker external port (#1490)

Make it possible to use env var EXTERNAL_PORT for specifying docker external port.
googledatalab/datalab,googledatalab/datalab,googledatalab/datalab,craigcitro/datalab,yebrahim/datalab,parthea/datalab,googledatalab/datalab,yebrahim/datalab,parthea/datalab,craigcitro/datalab,googledatalab/datalab,craigcitro/datalab,craigcitro/datalab,yebrahim/datalab,yebrahim/datalab,craigcitro/datalab,parthea/datalab,parthea/datalab,parthea/datalab,yebrahim/datalab,parthea/datalab,googledatalab/datalab