Reduce docker build context size by ignoring node_modules

node_modules can get large and adds time to the context upload to the
Docker engine. In our development environment, node_modules is mounted
from the host with the rest of the application source and will always be present.

.dockerignore

@@ -5,3 +5,7 @@
 log
 tmp
 vendor
+
+# For development build efficiency.
+# Consider restoring this if needed for a production build.
+node_modules