feat(ci, email): configurable SMTP SSL and Docker image build/push workflow

[VargaJoe]

Summary

This PR introduces two independent changes:

1. Configurable SMTP SSL — the EnableSsl flag on SmtpClient is now driven by configuration instead of being hard-coded, so deployments that target SMTP servers without TLS (e.g. internal relays) can opt out.
2. Docker image CI workflow — adds a GitHub Actions workflow that builds and publishes the application's Docker image to Docker Hub (sensenetcsp/sn-auth) on pushes to the main branches, validates the build on pull requests, and exposes a manual trigger for ad-hoc builds from any branch.

Changes

Email / SMTP

• EmailSettings.EnableSsl (bool, default true) added.
• EmailService now reads EnableSsl from EmailSettings instead of hard-coding true.
• appsettings.json extended with the new EnableSsl key (default true, preserves existing behavior).

CI / Docker

• New workflow: .github/workflows/docker-image.yml.
• Triggers:
  • push to develop, main, master → build and push image.
  • pull_request (non-draft) targeting develop, main, master → build only (validation, no push).
  • workflow_dispatch → manual run from any branch, with optional push_image and custom_tag inputs.
• Tagging strategy (via docker/metadata-action):
  • :<branch-name> for any push or PR build.
  • :latest only when pushing to main or master (independent of which branch is set as default in GitHub).
  • :preview only when pushing to develop.
  • :pr-<number> for pull request builds.
  • :<custom_tag> when supplied via manual dispatch.
• Build context is the repository root; Dockerfile path is src/sn-auth/Dockerfile.
• DockerHub login uses the existing organization secrets DOCKERHUB_USERNAME and DOCKERHUB_TOKEN, and only runs when an actual push is going to happen.

Behavior notes

• EnableSsl defaults to true, so existing deployments are unaffected unless they explicitly opt out.
• The Run workflow button (manual dispatch) only becomes visible in the GitHub Actions UI after this workflow lands on the default branch.

Testing

• PR build will execute the workflow against this branch via the pull_request trigger, validating that the Docker image builds end-to-end without pushing to the registry.
• SMTP change is config-only and preserves the previous default behavior.