Allow reference agent to launch Dockerized agents

The repository could mention that an agent is available as a Docker image, instead of saying that it's already running at a certain URL. We could allow for two kinds of search results. For example, "agent is running already and its card is available from this URL" could look like:

{
  "name": "...",
  "description": "...",
  "a2a_card_url": "http://...."
}

Alternatively, the result could mean "agent is available as a Docker image that you need to run yourself":

{
  "name": "...",
  "description": "...",
  "docker_image": "docker.io/..."
}

In that case, the reference agent should be able to run the Docker image itself while making it listen at an arbitrary open port and talk to it directly.

Note: we would also want the search results to have more than just the card URL. We should look at D2.1 for all the information that should be returned by the repository for a given agent.