Make SAVE IMAGE --push contextual within RUNs

[dchw]

This should allow the changes effected by a RUN --push to show in an image, when a SAVE IMAGE --push follows. SAVE IMAGE --push will not save locally unless it is the first -push command.

[vladaionescu]

The changes in builder make sense. I don't fully understand the changes in converter - so I left some comments.

[dchw]

Merged in, and tested with --enable-ast as well. I can add tests for that if needed? We would need to remove them when the AST is GA.

[vladaionescu]

Looks good - minor comment.

Let's merge this after the release.

[dchw]

This is the other part needed for #586.