Cache control

[maowtm]

I had a discussion with @barr, and he suggested to use operators that define conditions of cache invalidation. We were thinking of something like that:

• ::no_cache always invalidate the cache
• ::invalidate_cache_if_before(CMD) invalidate cache if CMD, executed before the layer is applied, returns non-exit code.
• ::invalidate_cache_if_after(CMD) invalidate cache if CMD, executed after the layer is applied, returns non-exit code.
• ::invalidate_cache_if_changed(PATH) invalidate cache if the file/directory changes after the layer is applied.

This is just a rough sketch of the API.

Originally posted by @mechtaev in #57 (comment)

Or maybe something like ::invalidate_cache_if_different(CMD_BEFORE, CMD_AFTER)

Originally posted by @mechtaev in #57 (comment)

• Implement the builtin
• Implement for buildkit
• Implement for Dockerfile (things other than a simple ::no_cache may not be possible)