[WIP] Add generic git task - with alternative auth

[jlpettersson]

Changes

The existing git-clone task is very opinionated and custom. This task use a generic git image. The plan is to add tasks for git-add, git-commit and git-push as well.

This commit also shows an alternative way to setup authentication. This authentication is independent of Tekton Pipeline Auth documentation and does not use annotations on Secrets and no need for custom credential initialization process and does not copy or move any sensitive values, but mount them using a projected volume.

Related to tektoncd/pipeline#2343

/kind feature

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[tekton-robot]

Hi @jlpettersson. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[jlpettersson]

both secret names should be params in a proper implementation.

  params:
  - name: known-hosts-secret
    type: string
    description: secret name of the SSH known_hosts file
  - name: private-key-secret
    type: string
    description: secret name of the SSH private key file

and in use

    projected:
      defaultMode: 0400
      sources:
      - secret:
          name: $(params.known-hosts-secret) 
      - secret:
          name: $(params.private-key-secret)

[ghost]

Is this a limitation of Tekton at the moment - params can't be used as variables in volumes?

[jlpettersson]

Yes, it is the Secret Name within a projected volume, were we don't have implemented variable substitution. But as the commit is implemented now, it is working. But for proper support, we should add variable substitution here, so that end-users can name the secrets how they want.