feat(dubboctl): add support for using CN mirror of buildpacks toolchain

[yin1999]

What is the purpose of the change

Part of #48 (may fix it). Considering that creating custom builders will increase maintenance costs, providing a mirror for the general builder (from Paketo Buildpacks) for users in mainland China is a lower-cost approach.

Brief changelog

• feat(dubboctl): add support for using CN mirror of buildpacks toolchain
• chore(dubboctl): add a comment about using CN mirror in the generated dubbo.yaml

Verifying this change

Using the following command to compile and build the dubboctl

go build -ldflags='-s -w -buildid=' -trimpath -o bin/dubboctl ./app/dubboctl/

And than using the dubboctl command to build a image (create any project of Golang, or Java) with the dubbo.yaml config:

# dubbo.yaml
runtime: java # or 'go' for golang project
image: test-server:latest
created: 2020-03-18T07:32:39.8834Z
build:
  buildEnvs:
  - name: BP_DEPENDENCY_MIRROR_GITHUB_COM # for java runtime
    value: mirror.example.com # may provide after we create the github release mirror
  - name: GOPROXY # for Golang runtime
    value: https://goproxy.cn,direct
  # Using the CN mirror for builders and lifecycle images
  cnMirror: true

/path/to/dubboctl build generate --image test-http-server:latest --push=false --force

CheckList

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[yin1999]

https://docker.aityp.com/ does not support latest tag, using tag mapping to resolve it.

[yin1999]

The buildEnvs configured in dubbo.yaml would be shadowed.