Split PRs into a dedicated workflow

.github/workflows/build-image.yaml

@@ -0,0 +1,15 @@
+name: Test Build
+
+on: [pull_request_target]
+
+jobs:
+  build-image:
+    name: Build Image
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+
+      - name: Build Image
+        run: make build

.github/workflows/publish-image.yaml

@@ -1,17 +1,16 @@
-name: Publish Image
+name: Build & Publish Image
 
 on:
   push:
     branches: ['master']
     tags: ['*']
-  pull_request:
-    branches: ['master']
 
 env:
   DOCKER_HUB_USER: phlak
 
 jobs:
-  build-container:
+  build-and-publish-image:
+    name: Build & Publish Image
     runs-on: ubuntu-latest
 
     steps: