Custom MONGODB_REPO_URL doesn't take MDB_IMAGE_TYPE into account

[vermut]

Duplicate of #1638 which is quite real and affecting air-gapped installs. Just to keep in "Open"

[vermut]

I did some digging and this is a direct consequence of #1271

I don't know why it is implemented in such manner that makes it impossible to use repo mirrors.

While "some customers" can still achieve backwards compatibility by explicitly setting image-type to null.