Generate datestamps during docker bake and not on merge

[klutchell]

This more accurately represents when the image was published, and not when the bake files were last updated.

Change-type: minor

[klutchell]

Did a manual run and checked the Docker Bake definition from the logs, seems to be working:

  {
    "group": {
      "default": {
        "targets": [
          "aarch64-alpine-3-16-run"
        ]
      }
    },
    "target": {
      "aarch64-alpine-3-16-run": {
        "attest": [
          "type=provenance,disabled=true"
        ],
        "context": "aarch64/alpine/3.16/run",
        "dockerfile": "Dockerfile",
        "tags": [
          "docker.io/balenalib/aarch64-alpine:3.16-run-20241112",
          "docker.io/balenalib/aarch64-alpine:3.16-run",
          "docker.io/balenalib/aarch64-alpine:3.16-20241112",
          "docker.io/balenalib/aarch64-alpine:3.16"
        ]
      }
    }
  }