pytorch: fix dockerize and deduplicate build logic

[jondea]

Add missing USERNAME variable from dockerize.sh, and use dockerize.sh in build.sh instead of duplictaing the build invocation. This needed the addition of a --build-only flag for dockerize.sh, but it means the existing behaviour doesn't need to change.

[nSircombe]

It looks fine.

Could you just confirm how many layers are in the finished docker image though?
...it shouldn't have introduced any new ones, but I want to make sure.
There should be 4 in total starting with the Ubuntu base image I believe.

[jondea]

Yep, I think so

docker image inspect toolsolutions-pytorch:latest
...
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:e437eb226c5d6bbd2c93a89e3708b0c005e0132aa682f109591fca659de658d1",
                "sha256:3d23d3bd11c0cdff5a178392d374628ecfc3bd6b0d3a4ad671b91618e59a6d4f",
                "sha256:2432bcd10674ea03db437b42992588d65b9153ef75dd18ef6d0b2f1ece732765",
                "sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
            ]
        },
...