Overhaul, update, reduce logging

[isobit]

Overhauls dockerdog to modernize the codebase and simplify configuration.

Modernize dependencies and build

• Switch from vendored dependencies to Go modules (go.mod/go.sum)
• Replace fsouza/go-dockerclient with the official docker/docker client library
• Replace flag with isobit/cli for argument parsing
• Replace the JSON config file with repeatable -a CLI flags for mapping Docker event attributes to StatsD tags
• Replace the Docker-based build (Dockerfile) with a static binary build via Makefile
• Add fmt, lint, test, and check Make targets

Simplify event handling

• Hardcode the set of tracked container/image events and their per-event attribute tags (e.g. exitCode on die, signal on kill) instead of requiring a JSON config file
• Use the Docker events API server-side filter so only relevant event types are received
• Move the "ignoring unknown event" log message behind a -debug flag to reduce noise in production

Update README

• Replace the plain event listing with a table showing each metric and its tags
• Document the new -a flag for mapping arbitrary event attributes to tags