Sometimes you need custom code in Docerfiles to solve for example
proxy problems or preconfigure base image in some specific way.
With this commit you can insert content of custom file to the beggining
of base Dockerfile to provide these changes
Partially-implements: bp rhel-based-image-support
Partially-implements: bp add-proxy-to-dockerfiles
Partially-implements: bp build-script
Change-Id: Idbfec764aad627f5c00dc09c4854d5fa1c5ac9cf
598fe7bad1