Add Docker build and publish workflow

[ravjotbrar]

Description

Adds a GitHub Actions workflow that builds and publishes the Dockerfile.app image to DockerHub, GHCR, and ECR Public.

Publish behavior:

• Nightly cron (0 0 * * *) → publishes as edge tag across all registries
• v tag push* (e.g. git push origin v1.0.0) → publishes as latest + the version number (e.g. 1.0.0)
• PRs / workflow_dispatch → builds only, no push

Registries:

• DockerHub
• GHCR — gated on github.repository == 'valkey-io/valkey-admin'
• ECR Public