add dockerfile input for monorepos

Lets a caller point at a non-default Dockerfile while keeping the
build context broader. Needed for repos with multiple binaries sharing
internal packages (api/Dockerfile + worker/Dockerfile, build context
at repo root). Empty default preserves prior behavior.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -27,6 +27,7 @@ the same tag.
 |---|---|---|---|
 | `image` | yes | — | Full image name without tag (e.g. `code.fritzlab.net/fritzlab/chrony`). |
 | `context` | no | `.` | Docker build context. |
+| `dockerfile` | no | `Dockerfile` (in context) | Path to the Dockerfile, relative to the context (or absolute under `$GITHUB_WORKSPACE`). Use for monorepos where the build context is the repo root but the Dockerfile lives in a subdir, e.g. `dockerfile: api/Dockerfile`. |
 | `build-args` | no | — | Multiline `KEY=VALUE` build args. |
 | `smoke-test` | no | — | Shell command run after build. `$IMAGE` is set to `<image>:<run_number>`. Non-zero exit fails the action. |
 

action.yaml

@@ -8,6 +8,14 @@ inputs:
     description: Docker build context path
     required: false
     default: .
+  dockerfile:
+    description: |
+      Path to the Dockerfile, relative to the context (or absolute under
+      $GITHUB_WORKSPACE). Defaults to "Dockerfile" inside the context.
+      Use this for monorepos where the build context is the repo root but
+      the Dockerfile lives in a subdir (e.g. dockerfile: api/Dockerfile).
+    required: false
+    default: ''
   build-args:
     description: Multiline KEY=VALUE pairs passed to docker build
     required: false
@@ -29,6 +37,7 @@ runs:
       uses: docker/build-push-action@v6
       with:
         context: ${{ inputs.context }}
+        file: ${{ inputs.dockerfile }}
         push: false
         load: true
         provenance: false