Update docs/github-action.md

Co-authored-by: Copilot <[email protected]>

Author: tgenov

docs/github-action.md

@@ -142,9 +142,9 @@ The [`devcontainers/ci` action](https://github.com/marketplace/actions/devcontai
 | cacheFrom                 | false    | Specify additional images to use for build caching                                                                                                                                                                                             |
 | noCache                   | false    | Builds the image with `--no-cache` (takes precedence over `cacheFrom`)                                                                                                                                                                         |
 | cacheTo                 | false    | Specify the image to cache the built image to                                                                                                                                                                                             |
-| platform                  | false    | Platforms for which the image should be built. If omitted, defaults to the platform of the GitHub Actions Runner. Multiple platforms should be comma separated.                                                                                |
-| platformTag               | false    | Tag suffix for this platform build (e.g., `linux-amd64`). Used in matrix builds to push per-platform images that are later merged into a multi-arch manifest.                                                                                 |
-| mergeTag                  | false    | Comma-separated list of platform tags to merge into a multi-arch manifest (e.g., `linux-amd64,linux-arm64`). Used in the merge job after matrix builds complete.                                                                              |
+| platform                  | false    | Platforms for which the image should be built. If omitted, defaults to the platform of the GitHub Actions Runner. Multiple platforms should be comma separated. Ignored when `platformTag` is set for a native per-platform build, or when `mergeTag` is set for a merge-only job. |
+| platformTag               | false    | Tag suffix for this platform build (e.g., `linux-amd64`). Used in matrix builds to push per-platform images that are later merged into a multi-arch manifest. When set, this job is treated as a native per-platform build and the `platform` input is ignored.                     |
+| mergeTag                  | false    | Comma-separated list of platform tags to merge into a multi-arch manifest (e.g., `linux-amd64,linux-arm64`). Used in the merge job after matrix builds complete. When set, the job only merges existing per-platform images and does not build; `platform` and `platformTag` are ignored. |
 
 ## Outputs