fix(Dockerfile.alpine): gcloud auth naming and targetarch from alpine

[keg]

Hello, thanks for creating this tool. It's been helpful for our CI/CD processes.

I am submitted this PR with two fixes that we found helpful.

• Change bld to use TARGETARCH as we would want the from source arch to be same as the target.
• Changing google auth helper to be named /usr/bin/docker-credential-gcloud. When using the gcloud helper command gcloud auth configure-docker <gcr-or-gar> the resulting docker config file is looking for docker-credentials-gcloud.

Example docker config:

{
  "auths": {},
  "credsStore": "desktop",
  "credHelpers": {
    "asia.gcr.io": "gcloud",
    "eu.gcr.io": "gcloud",
    "gcr.io": "gcloud",
    "marketplace.gcr.io": "gcloud",
    "staging-k8s.gcr.io": "gcloud",
    "us.gcr.io": "gcloud",
  },
  "currentContext": "desktop-linux"
}

[estesp]

Thanks so much! Good to hear the tool is useful for you. Changes look totally reasonable, but can you sign your commit with a Signed-off-by: line for the DCO check?

[keg]

Thanks for the contribution!

Thank you and thanks for the speedy merge!