fix(express): remove hardcoded dockerfile labels

[louib]

Removing the hardcoded labels from the Dockerfile will remove the need to create a new commit when publishing a new version of bitgo/express, which in turn will make the release process more streamlined.

I'm also removing explicit references to podman from the build scripts since docker is used to produce the official releases of bitgo/express. The CI job that builds the docker image on PRs continues to use podman since it is proving to be significantly faster than docker (10 minutes instead of docker which errors out after 15 minutes). Unless we want to use a bigger runner, this is the best compromise I can think of.

I was able to verify that the labels are still populated correctly:

            "Labels": {
                "created": "2025-09-26T20:17:27Z",
                "git_hash": "69e6314ac0a27b9d09dc117c9ee78bf5f6bf4bc0",
                "version": "15.1.0"
            }

Ticket: VL-3498