--as-dockerfile doesn't read builder image labels labels

It seems that s2i build --as-dockerfile doesn't read the builder image labels when generating the Dockerfile, especially the io.openshift.s2i.* ones.

As an example, see this gist. The image use in there registry.access.redhat.com/redhat-openjdk-18/openjdk18-openshift.

skopeo inspect docker://registry.access.redhat.com/redhat-openjdk-18/openjdk18-openshift | grep io.openshift.s2i
        "io.openshift.s2i.destination": "/tmp",
        "io.openshift.s2i.scripts-url": "image:///usr/local/s2i",

When trying to build the generated Dockerfile from this builder, it fails as it is looking at it tries to execute a RUN on /usr/libexec/s2i/assemble, which doesn't exists in that image.

STEP 6: USER 1001
--> 496bfb3d9130701e154304c2b444788fc6110780b26adc71620315b3d440a8b2
STEP 7: RUN /usr/libexec/s2i/assemble
/bin/sh: /usr/libexec/s2i/assemble: No such file or directory
subprocess exited with status 127
subprocess exited with status 127
error building at STEP "RUN /usr/libexec/s2i/assemble": exit status 127

Related to https://github.com/openshift/source-to-image/issues/966#issuecomment-495169765.

cc @siamaksade @rhuss

Start with the s2i build --as-dockerfile entry point and the handling of builder-image labels, reproducing the issue with the image and skopeo inspect example in this report. Done means the generated Dockerfile uses the builder's io.openshift.s2i.* labels, including its scripts location, and builds without trying to run the wrong assemble path.