fix: label container images correctly

[enocom]

Fixes #2060

[jackwotherspoon]

whoops, nice catch!

[enocom]

Running, docker inspect <image>:

Before:

"Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "65532",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt"
            ],
            "Cmd": null,
            "Image": "",
            "Volumes": null,
            "WorkingDir": "/home/nonroot",
            "Entrypoint": [
                "/cloud-sql-proxy"
            ],
            "OnBuild": null,
            "Labels": null
        },

After:

"ContainerConfig": {
            "Hostname": "7b4a7a6a0748",
            "Domainname": "",
            "User": "65532",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt"
            ],
            "Cmd": [
                "/bin/sh",
                "-c",
                "#(nop) ",
                "ENTRYPOINT [\"/cloud-sql-proxy\"]"
            ],
            "Image": "sha256:32ef7b65ede49d79f7827090bffb44625274298fb0f919f46272569f4d062ef1",
            "Volumes": null,
            "WorkingDir": "/home/nonroot",
            "Entrypoint": [
                "/cloud-sql-proxy"
            ],
            "OnBuild": null,
            "Labels": {
                "org.opencontainers.image.source": "https://github.com/GoogleCloudPlatform/cloud-sql-proxy"
            }
        },