ci: set up publishing docker image

[kantord]

Tested by temporarily making the GH action run on the PR branch instead of on tags. The image was successfully published: https://github.com/stacklok/toolhive-cloud-ui/pkgs/container/toolhive-cloud-ui

To pull the image locally, you need to do this:

docker pull ghcr.io/stacklok/toolhive-cloud-ui:latest

if that does not work, you need to make sure that you are locally authenticated and that your token has permissions to read packages:

gh auth refresh -h github.com -s read:packages
gh auth token | docker login ghcr.io -u kantord --password-stdin

I tested it by running it like so:

docker run -d -p 3002:3000 --name toolhive-test ghcr.io/stacklok/toolhive-cloud-ui:latest

and it works fine!

[Copilot]

Pull Request Overview

This PR introduces a GitHub Actions workflow to automate building and publishing Docker images to GitHub Container Registry (GHCR) when version tags are pushed.

• Adds automated Docker image publishing triggered by version tags (v*)
• Configures authentication to GHCR using GitHub's built-in token
• Sets up dual tagging strategy with both latest and version-specific tags

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