Add a GitHub Action to do a test build for both templates

[alexellis]

Add a GitHub Action to do a test build for both templates

Expected Behaviour

We should create a function with and without a Go template with both templates during CI and validate that they can build with docker or faas-cli build.

Current Behaviour

Manual testing

Possible Solution

Add a bash / contrib script to do this and a GitHub Action to run on push/pr.

Context

Backwards compatibility, preventing issues for users.

[alexellis]

/add label: good first issue, help wanted