Reduce layer count in Docker image:

[crast]

• move the bzip2 file and extraction into the scratch image
• combine commands and ENV directives to make less layers

[Cobrijani]

ENV variables are not making additional layers. https://docs.docker.com/develop/develop-images/dockerfile_best-practices/

[niondir]

Reducing the layers is good anyway. Since merging the ENV lines should not have any side effects we can leave it for now.