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/build.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+set -e -x
+
+rm -rf ./build
+mkdir ./build
+
+(
+    cd build
+
+    cp ../../../../nebula .
+    cp ../../../../nebula-cert .
+    cp ../*.yml .
+
+    ./nebula-cert ca -name "Smoke Test"
+    ./nebula-cert sign -name "lighthouse1" -ip "192.168.100.1/24"
+    ./nebula-cert sign -name "host2" -ip "192.168.100.2/24"
+    ./nebula-cert sign -name "host3" -ip "192.168.100.3/24"
+)
+
+docker build -t nebula:smoke .