When building from Dockerfile, add an option to select a named stage as a build target

[radekg]

Today, firebuild assumes that an unnamed stage exists in the Dockefile and will not build if it does not exist. However, having a named stage as the only stage in the Dockerfile is perfectly fine. There are two options possible and both can be supported equal:

1. if the Dockerfile contains a single stage and the stage is named, build it
2. if there are multiple stages and no final unnamed stage, allow specifying the --dockerfile-stage=string and build that stage as the rootfs