Hacky container.v1 prototype

[vvuk]

Here's a horrible hack of how container.v1 could maybe be supported, to fix to fix #264. I think
the difference is that .v1 can have either an image entry to behave like v0, or a build entry
to do a build first. This is a complete hack, as it doesn't handle v1 with image, but I think
Aspire writes those out as v0 anyway.