feat: support specifying custom image names

[miguelvr]

Description

Currently, the only way to specify the image name is to rename a folder. Sometimes, the folder name has a generic name that makes sense only within the project context, but not as a global name.

There are 2 workarounds for this, but they both have their disadvantages:

1. use the full package name for image naming (e.g. github.com/company/repo/cmd/name) → I find this extremely cumbersome, and not particularly readable or easy to manipulate downstream
2. set KO_DOCKER_REPO to something like registry/repo → this would lead to an image name like repo/name

I find myself using option 2) the most, but I would like to have the freedom to specify custom image names.

Proposal

Note

Flag name and config option name are open to discussion

Use the build id to set the name of the image.

ko build --use-id ./cmd/name

alternatively, allow the user to specify this flag in the configuration, and just use ko build ./cmd/name

builds:
- id: foo
  nameFromID: true
  main: ./cmd/name

This would lead to an image named foo, despite being located in cmd/name

Let me know if the project is open to this proposal, and I'm happy to contribute it

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