Github Actions: Smoke test

This change adds a new Github Action, a 3 node smoke test. It starts
three docker containers (one lighthouse and two standard nodes) and
tests that they can all ping each other. This should hopefully detect
any basic runtime failures in PRs.

.github/workflows/smoke.yml

@@ -0,0 +1,34 @@
+name: smoke
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+jobs:
+
+  smoke:
+    name: Run 3 node smoke test
+    runs-on: ubuntu-latest
+    steps:
+
+    - name: Set up Go 1.13
+      uses: actions/setup-go@v1
+      with:
+        go-version: 1.13
+      id: go
+
+    - name: Check out code into the Go module directory
+      uses: actions/checkout@v1
+
+    - name: build
+      run: make
+
+    - name: setup docker image
+      working-directory: ./.github/workflows/smoke
+      run: ./build.sh
+
+    - name: run smoke
+      working-directory: ./.github/workflows/smoke
+      run: ./smoke.sh
+
+    timeout-minutes: 10