So something like:

archive(
    name="web-tarball",
    packages=[
        "src/python/org/services/silly_web_backend:silly_web_backend-dist",
        "src/python:org-libs"
    ],
    format="tar.gz"
)

docker_image(
    name="web_backend",
    dependencies=[
        ":msg",
        ":web-tarball",
        ":image_scripts"
    ]
)

Which means you have access to

<http://path.to|path.to>.docker.target/web-tarball.tar.gz

in the build context