feat(anubis): add /healthz route to metrics server

Also add health check test for Docker Compose and update documentation
for health checking Anubis with Docker Compose.

Signed-off-by: Xe Iaso <me@xeiaso.net>

test/healthcheck/test.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+set -eo pipefail
+
+export VERSION=$GITHUB_COMMIT-test
+export KO_DOCKER_REPO=ko.local
+
+set -u
+
+(
+  cd ../.. && \
+  ko build --platform=all --base-import-paths --tags="latest" --image-user=1000 --image-annotation="" --image-label="" ./cmd/anubis -L
+)
+
+docker compose up -d
+
+attempt=1
+max_attempts=5
+delay=2
+
+while ! docker compose ps | grep healthy; do
+  if (( attempt >= max_attempts )); then
+    echo "Service did not become healthy after $max_attempts attempts."
+    exit 1
+  fi
+  echo "Waiting for healthy service... attempt $attempt"
+  sleep $delay
+  delay=$(( delay * 2 ))
+  attempt=$(( attempt + 1 ))
+done
+
+docker compose down