Update to new image in Dockerfile

[the-jeffski]

Update to the Docker file process to use a smaller image file when building and run in two stages to reduce the size even further once the compiles are completed.

[the-jeffski]

That's fair. I copied that from my build script but actually just mentioned a hint to do a prune when building a new image.

[ThomDietrich]

Looks great! Thanks for your contribution :)