Add Docker configuration and startup script for Plakar UI

[guigui42]

The goal is to make it easy to self host plakar ui using Docker.

Docker support for Plakar UI:

• .dockerignore: Added a .dockerignore file to exclude unnecessary files from the build context, such as Git files, documentation, test files, CI/CD files, and temporary files.
• Dockerfile: Created a multi-stage Dockerfile to build and run the Plakar UI application. The build stage compiles the application using Go, while the runtime stage sets up a minimal Alpine-based environment with a non-root user for security, health checks, and default startup commands.
• docker/docker-compose.yml: Added a Docker Compose configuration to simplify running the Plakar UI in a container. It includes service definitions, port mappings, volume mounting for the repository, environment variable configuration, and automatic restart policies.
• docker/start-plakar-ui.sh: Introduced a startup script to launch the Plakar agent and UI server within the container. The script ensures proper initialization and error handling.

Next step would be to create a Workflow to build the image and push to GitHub directly and make the docker image available to everyone whenever there is a new Release. (if needed I can also take care of it )