Document docker action usage

[aplr]

Quite a edge-case probably, but an easy fix if documented.

I'm running cloud-sql-proxy in combination with tailscale to access my postgres instance w/ private address in Google Cloud.

The proxy exposes the connection on a port on the github runner's localhost.

As this action runs in a docker container itself, connecting to localhost, does of course not connect to the github runner's "localhost".

The fix is using 172.17.0.1 as a host, instead of localhost:

- name: Deploy Atlas Migrations
  uses: ariga/atlas-deploy-action@v0
  env:
    DB_HOST: "172.17.0.1"
    DB_PORT: "1234"
    DB_USERNAME: pg
    DB_PASSWORD: pg
    DB_NAME: pg
  with:
    url: postgres://${{ env.DB_USERNAME }}:${{ env.DB_PASSWORD }}@${{ env.DB_HOST }}:${{ env.DB_PORT }}/${{ env.DB_NAME }}?search_path=public&sslmode=disable

Probably it'd be good to document that this action is running in a docker container, as I was looking too long on my end.
And also, how to connect to localhost then.

[giautm]

honestly, I don't know how to connect to localhost from inside of the action yet. I just write to update you that we already supported for CloudSQL IAM authenticator using the new data source: https://atlasgo.io/atlas-schema/projects#data-source-gcp_cloudsql_token

[aplr]

I'm actively running migrations using ariga/atlas-deploy-action, connecting to runner's localhost by using 172.17.0.1 as the host. Working fine as far as I can tell :)