support multiple BUILDPACK_VOLUMES - issue #38

BUILDPACK_VOLUMES doesn't support multiple volumes, as currently written.

it's not super clear from upstream how its supposed to work as there's mentions of commas for multiple values, but testing in the pipelines on this MR suggests --volume has to be passed multiple times. See #38 (comment 731233862) for the same conclusion.

This change should allow BUILDPACK_VOLUMES to be passed with a single volume as before, or multiple volume definitions separated by a pipe |

BUILDPACK_VOLUMES: "/somewhere/1:/volumes/1:ro|/elsewhere/2:/volumes/2:ro|/mnt/foo:/volumes/3:ro|/mnt/bar:/volumes/4:ro"

closes #38 (closed)