proposal: Flag for disabling Earthly internal labels

[3manuek]

Use case

Right now, Earthly includes a set of labels at SAVE IMAGE that are not related to the build itself:

    labels := map[string]string{
        "dev.earthly.version":  version.Version,
        "dev.earthly.git-sha":  version.GitSha,
        "dev.earthly.built-by": version.BuiltBy,
    }

Code

A flag like SAVE IMAGE --disable-earthly-labels might work. Also, a feature flag might need to be added.

Expected Behavior

It is expected that the user can control whether these labels are added or not to the produced image.

[alexcb]

2721c63 is a good example which shows what's needed to introduce a new feature flag.

[3manuek]

I started the patch at #4084. I don't see how to make the version feature to work even tho I added it, probably I'm missing some compilation flag cc @alexcb .