Clearly indicate the demarcation of build stage in the build output

[LaurentGoderre]

When using multi-stage build, thew output does not clearly identify where one stage ends and another begin. I need to run the first stage image after the build is done to extract build related info (such a build cache) but findng the imgage ID is difficult because the output is just like any other step.

Here is the output of one of my multi-stage edge

Removing intermediate container 535f930c06dc
 ---> 3e491620080f
Step 7/14 : FROM alpine:3.4

It would be nice if instead it would say:

Removing intermediate container 535f930c06dc
Successfully built 3e491620080f (stage: 0)
Step 7/14 : FROM alpine:3.4

or for named ones:

Removing intermediate container 535f930c06dc
Successfully built 3e491620080f (stage: build)
Step 7/14 : FROM alpine:3.4

[thaJeztah]

Continued in moby/buildkit#521, and implemented in BuildKit in moby/buildkit#549