Add CI check for license headers

Otherwise no one is ever going to add them.

.github/main.workflow

@@ -6,6 +6,7 @@ workflow "Pull Request" {
     "vet",
     "lint",
     "test",
+    "check-licenses",
   ]
 }
 
@@ -32,3 +33,8 @@ action "test" {
   }
   runs = ["go", "test", "-race", "-cover", "./..."]
 }
+
+action "check-licenses" {
+  uses = "docker://golang:1.12.6"
+  runs = ["./script/check-licenses.sh"]
+}

script/check-licenses.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+function list_go_files {
+    srcdir=$(realpath $(dirname "${BASH_SOURCE[0]}")/..)
+    for f in $(find "$srcdir" -name '*.go' -and -not -path "$srcdir/vendor/*") ; do
+        echo "$f"
+    done
+}
+
+function has_license {
+    head -n2 "$1" | grep -q 'Copyright .... DigitalOcean'
+}
+
+ret=0
+for f in $(list_go_files) ; do
+    if ! has_license "$f" ; then
+        echo "$f is missing license header"
+        ret=1
+    fi
+done
+
+exit $ret