Collect the logs of the local Docker containers

- docker.go follows every running container through the Docker API
  (events + logs with follow), resumes after a restart from the last
  position saved in /data/docker-state.json, reads DOCKER_BACKFILL (1h)
  of history for new containers, strips terminal color codes and guesses
  the severity from the line (JSON, logfmt, [ERROR], ERROR ...).
- Logs carry source_type=docker, container, container_id, image,
  compose_project, compose_service and stream; host is the Docker host.
- Settings > Sources: one switch per container (grouped by compose
  project), enable/disable all, follow new containers automatically.
  Choices are saved per compose service in /data/docker.json.
- Source filter (syslog / docker) in the filter bar and the live view.
- docker-compose: read-only docker-socket-proxy; Logstream and the proxy
  are labelled logstream.exclude=true and never collected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -15,3 +15,7 @@ DNS_SERVER=
 ALLOW_PURGE=true
 # Maximum number of rows in a CSV export
 EXPORT_MAX=100000
+# Collect the logs of the Docker containers of this machine (on/off)
+DOCKER_LOGS=on
+# History read from a container seen for the first time (e.g. 30m, 1h, 24h; 0 = only new lines)
+DOCKER_BACKFILL=1h