Dockerfile in build yml and CLI; tag templates

[deitch]

- What I did

Added two build options to lkt pkg build

• dockerfile can be specified both in build.yaml and as --dockerfile CLI flag
• added option to override the tag when building or show-tag via --tag <tag>

For the dockerfile specification, this fits perfectly with the linuxkit philosophy. Everything is reproducible as it is files on the disk. The same way you can specify a different build.yml, you can reference the specific dockerfile.

For the tag, we allow lots of flexibility in naming the generated image. This now lets you specify the actual tag, including using templates, e.g. --tag "version2-{{.Hash}}. Right now, only .Hash is supported, but we could add a more things if they fit.

- How I did it

Changes

- How to verify it

I tested manually, and added tests

- Description for the changelog

Dockerfile selection support, tag flexibility