Function builds using buildpacks

[jldec]

This issue proposes to replace the existing default build strategy in the riff CLI with Buildpacks v3 and depends on availability of v3 buildpacks support in a Knative buildpack build template.

Since v0.1.0, the riff CLI has used riff-build to configure function builds with a riff-buildtemplate. This first generates a Dockerfile from a template in the function invoker yaml, together with parameters from the function configuration. The Dockerfile is then handed off to Kaniko to build the image relying on the same process as the Knative Kaniko buildtemplate.

Using a buildpack build template should provide riff function developers on Knative with the same user experience as the existing Dockerfile based builds, but brings additional benefits such as more flexible day2 management of image layers. The pack CLI should also enable a localhost workflow, integrating with the riff CLI.

• v3 support in Knative buildpack build template #45
• modify riff CLI to build functions using buildpack build template
• buildpack v3 for node
• buildpack v3 for npm
• buildpack v3 for java
• buildpack v3 for command