Add configurable User-Agent via USER_AGENT env var

[doringeman]

Makes use of docker/compose@4bbc6c6 and docker/model-runner#101.

E.g.,

$ # terminal 1

$ DEBUG=1 MODEL_RUNNER_PORT=8080 make run 2>&1 | grep "with user agent: "

$ # terminal 2

$ cat test-compose-models.yaml
services:
  ai_runner:
    image: alpine
    models:
      - ai_model
models:
  ai_model:
    model: ai/llama3.2
    context_size: 1024
    runtime_flags:
      - "--no-prefill-assistant"

$ MODEL_RUNNER_HOST=http://localhost:8080 docker compose -f test-compose-models.yaml up
# in terminal 1 you'll see:
# time="2025-07-08T15:18:31+03:00" level=debug msg="Tracked index.docker.io/ai/llama3.2:latest latest
# with user agent: docker-model-runner docker-model-cli/dev compose/v2.23.3-812-g9e17a091b" component=metrics

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model run ai/smollm2 hi
# in terminal 1 you'll see:
# time="2025-07-08T15:18:38+03:00" level=debug msg="Tracked index.docker.io/ai/smollm2:latest latest
# with user agent: docker-model-runner docker-model-cli/dev" component=metrics

$ curl http://localhost:8080/engines/v1/chat/completions -X POST -H "Content-Type: application/json" -d '{
    "model": "dorin/smollm2",
    "messages": [
      {"role": "user", "content": "Capital of Romania?"}
    ]
  }'
# in terminal 1 you'll see:
# time="2025-07-08T15:18:50+03:00" level=debug msg="Tracked index.docker.io/ai/smollm2:latest latest
# with user agent: docker-model-runner curl/8.7.1" component=metrics