fix: docker build requires smtp host (#672)

set a default for smtp host and add an action
for testing docker builds on each pull request

apps/marketing/package.json

@@ -13,6 +13,7 @@
     "copy:pdfjs": "node ../../scripts/copy-pdfjs.cjs"
   },
   "dependencies": {
+    "@documenso/assets": "*",
     "@documenso/lib": "*",
     "@documenso/tailwind-config": "*",
     "@documenso/trpc": "*",