Envbuilder fails if lifecycle entries are in OCI metadata

[mason3263]

When a devcontainer is built lifecycle scripts can be included in the metadata. If this is the case envbuilder fails to unmarshal the metadata embedded in the OCI image. I believe envbuilder is trying to parse the metadata using the Spec struct in devcontainer.go file.

{
    "Name": "my-image",
    "Digest": "sha256:ad63b47f418820329b1f0aa0f707ad590cc6b63efdc9de86bdd76f1fa6c329b0",
    "RepoTags": [
        "latest"
    ],
    "Created": "2025-05-17T03:14:27.157348203Z",
    "DockerVersion": "",
    "Labels": {
        "devcontainer.metadata": "{\"postAttachCommand\":\"bash /scripts/entrypoint.bash\",\"containerEnv\":{\"DISPLAY\":\"${localEnv:DISPLAY}\",\"VISUAL\":\"code --wait\",\"EDITOR\":\"code --wait\",\"GIT_EDITOR\":\"code --wait\",\"CC\":\"/usr/bin/clang\",\"CXX\":\"/usr/bin/clang++\"}}",
        "io.buildah.version": "1.39.4",
        "org.opencontainers.image.ref.name": "ubuntu",
        "org.opencontainers.image.version": "22.04"
    },
...

[mason3263]

I looked into this a little further and adding UnknownFields map[string]interface{} `json:"-"` could potentially fix this by catching all unknown fields in a Map. Adding lifecycles do the spec would also fix this issue but imo unknown fields should be caught anyway to prevent issues in the future. I should be able to implement this sometime soon.