Add a GitHub Action to validate we can build the site

This is mostly here as a placeholder to get CI working -- I'm going
to be adding more validation shortly.

Author: PeterJCLaw

.github/workflows/validate.yml

@@ -0,0 +1,24 @@
+name: Validate
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.7
+        bundler-cache: true
+
+    - name: Build
+      run: bundle exec rake build

Rakefile

@@ -19,3 +19,7 @@ task :submodules => ['_sass/brand/.git']
 task :dev => [:dependencies, :submodules] do
   sh('bundle exec jekyll serve --drafts --config _config.yml')
 end
+
+task :build => [:dependencies, :submodules] do
+  sh('bundle exec jekyll build --config _config.yml')
+end