Add melange/apko build pipeline and Kubernetes manifests

- Add melange.yaml for distroless package build
- Add apko.yaml for minimal container image
- Add GitHub Actions workflow for melange/apko CI build
- Add Kubernetes manifests (deployment, service, configmap, secret, redis)
- Update Dockerfile, .gitignore, and docker-compose.yml

k8s/configmap.yaml

@@ -0,0 +1,17 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: apcupsd-client-config
+  namespace: apcupsd
+  labels:
+    app.kubernetes.io/name: apcupsd-client
+    app.kubernetes.io/part-of: apcupsd-client
+data:
+  REDIS_URL: "redis://redis:6379/0"
+  LOG_LEVEL: "info"
+  TZ: "UTC"
+  ALLOW_PRIVATE_IPS: "true"
+  TRUST_PROXY: "true"
+  RATE_LIMIT_ENABLED: "true"
+  ADMIN_USERNAME: "admin"
+  SESSION_MAX_AGE_SECONDS: "86400"