feat: Change to using composite run instead of Docker

https://docs.github.com/en/actions/creating-actions/creating-a-composite-run-
steps-action

Author: lcenchew

Dockerfile

@@ -10,12 +10,14 @@ inputs:
     description: 'Upstream repository owner/name. For example, exions/merge-upstream'
     required: true
   branch:
-    description: 'This branch to merge from and to. For example, master'
+    description: 'Branch to merge to. For example, master'
     default: 'master'
 
 runs:
-  using: 'docker'
-  image: 'Dockerfile'
-  args:
-    - ${{ inputs.upstream }}
-    - ${{ inputs.branch }}
+  using: "composite"
+  steps: 
+    - run: | 
+        git remote add -f upstream "https://github.com/${{ inputs.upstream }}.git"
+        git merge --ff-only upstream/${{ inputs.branch }}
+        git push 
+      shell: bash