Don't allow snapshots if docker images are parameterized

[NatalieEO]

Feature Request

Desired behaviour

We should not allow a version to be snapshotted if the docker images are parameterized. Should give user message as to why in order to discourage the practice.

┆Issue is synchronized with this Jira Story ┆fixVersions: Dockstore 2.X ┆friendlyId: DOCK-1202 ┆sprint: Backlog ┆taskType: Story

[aofarrel]

I propose taking this a step further. Docker tags are mutable, Docker digests are immutable. Snapshots should require a reference to a specific Docker digest for all referenced images. Be warned, this does have the disadvantage of flying in the face of the most common practice, but for your consideration: https://github.com/nodejs/docker-node/issues/649