Add local builds

This commit adds the ability for users to define functions to be built locally, instead of being pulled as a Docker image for a remote repository. The main reason this was added was to facilitate integration tests for faas-template changes, however this could be useful when a user wants to build a FaaS function based on an external Git repository. The user can either choose to build the function locally, or use a pre-build function. This is controlled via the flag in the piperci-releases file.