New flag --repo-prefix to backfill --default-repo functionality

Expected behavior

Previously I had been using --default-repo more like --repo-prefix this all came to a grinding halt when I attempted to use the below skaffold.yaml. When I ran it with

skaffold build --default-repo=abc.123.com

I expected to see:

building abc.123.com/bases/go/1.13

building abc.123.com/bases/go/1.14

Actual behavior

building abc.123.com/bases_go_1.13

building abc.123.com/bases_go_1.14

Information

• Skaffold version: v1.10.1
• Operating system: Linux
• Contents of skaffold.yaml:

apiVersion: skaffold/v1beta15
kind: Config

build:
  artifacts:
    - image: bases/go/1.13
      context: bases/go/multi
      docker:
        buildArgs:
          GO_VERSION:     "1.13"
          ALPINE_VERSION: "3.12"
    - image: bases/go/1.14
      context: bases/go/multi
      docker:
        buildArgs:
          GO_VERSION:     "1.14"
          ALPINE_VERSION: "3.12"

Proposal

We introduce a new flag --repo-prefix which doesn't mangle the image names for Docker Hub and simply applies the prefix.