Convert to a composite action

JavaScript actions have no way of supporting Node 18. We want to use
Node 18 to fix #33.

It seems that GitHub will never support `node18` for `actions/runner`,
instead jumping straight to Node 20.

action.yaml

@@ -29,6 +29,25 @@ inputs:
     required: true
     description: A list of files/directories to upload to Riff-Raff. An alternative to nesting `sources` inside the `config` input.
 
+# When `actions/runner` supports Node 18 or higher, this could be a Javascript action.
+# See https://github.com/guardian/actions-riff-raff/issues/33
 runs:
-  using: "node16"
-  main: "dist/index.js"
+  using: composite
+  steps:
+    - name: Setup node
+      uses: actions/setup-node@v3
+      with:
+        node-version-file: .nvmrc
+    - name: Action
+      run: node ${{ github.action_path }}/dist/index.js
+      shell: bash
+      env:
+        INPUT_APP: ${{ inputs.app }}
+        INPUT_CONFIG: ${{ inputs.config }}
+        INPUT_CONFIGPATH: ${{ inputs.configPath }}
+        INPUT_PROJECTNAME: ${{ inputs.projectName }}
+        INPUT_DRYRUN: ${{ inputs.dryRun }}
+        INPUT_BUILDNUMBER: ${{ inputs.buildNumber }}
+        INPUT_BUILDNUMBEROFFSET: ${{ inputs.buildNumberOffset }}
+        INPUT_STAGINGDIR: ${{ inputs.stagingDir }}
+        INPUT_CONTENTDIRECTORIES: ${{ inputs.contentDirectories }}