Add deploy to gh-pages for dev branch (#1305)

Signed-off-by: Matthew Peveler <matt.peveler@gmail.com>

.github/workflows/deploy.yml

@@ -32,3 +32,4 @@ jobs:
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         publish_dir: ./build
+        keep_files: true

.github/workflows/dev_deploy.yml

@@ -0,0 +1,36 @@
+name: Dev Deploy
+
+on:
+  push:
+    branches: [ 'dev' ]
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    env:
+      ruby-version: 2.5
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ env.ruby-version }}
+
+    - uses: actions/cache@v1
+      with:
+        path: vendor/bundle
+        key: gems-${{ runner.os }}-${{ env.ruby-version }}-${{ hashFiles('**/Gemfile.lock') }}
+
+    - run: bundle config set deployment 'true'
+    - run: bundle install
+
+    - run: bundle exec middleman build
+
+    - name: Deploy
+      uses: peaceiris/actions-gh-pages@v3.7.0-8
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        destination_dir: dev
+        publish_dir: ./build
+        keep_files: true