git-crypt unlock fails because `bash` doesn't exist

[sallar]

Hi,

I found this repo randomly but it has solved my issue. thanks for that.
But it took me a while to get it working because the git-crypt unlock KEY command was failing and saying git checkout fails because there is no bash. So this is how I solved this issue:

  - name: 'gcr.io/cloud-builders/gcloud'
    id: Fetch Key
    entrypoint: /bin/sh
    dir: './functions'
    args:
      - '-c'
      - |
        echo "${_GIT_CRYPT_KEY}" | base64 --decode > git-crypt-key

  - name: 'gcr.io/cloudkite-public/git-crypt'
    id: Decrypt
    dir: './functions'
    entrypoint: /bin/sh
    args:
      - '-c'
      - |
        apk add --no-cache bash && git-crypt unlock git-crypt-key

What do you think about adding the apk add --no-cache bash command to the Dockerfile? I can send a PR if you agree.