fix: docker build requires smtp host (#672)

set a default for smtp host and add an action
for testing docker builds on each pull request

.github/workflows/ci.yml

@@ -14,8 +14,8 @@ env:
   HUSKY: 0
 
 jobs:
-  build:
-    name: Build
+  build_app:
+    name: Build App
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -37,3 +37,16 @@ jobs:
 
       - name: Build
         run: npm run build
+
+  build_docker:
+    name: Build Docker Image
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 2
+
+      - name: Build Docker Image
+        run: ./docker/build.sh
+