docker: alpine base with openssh-client, mount SSH key + NET_RAW for scheduler

- Switch Dockerfile from distroless/static to alpine:3.21
- Install openssh-client-default in runtime image
- Mount SSH key in scheduler service (docker-compose)
- Add NET_RAW capability for ping checks
- Wire OIKOS_SSH_KEY_PATH and OIKOS_SSH_USER env vars in scheduler
- sshExec uses configured key path with StrictHostKeyChecking=no

docker-compose.yml

@@ -83,6 +83,11 @@ services:
       OIKOS_DATABASE_URL: postgres://oikos:${OIKOS_DB_PASSWORD:-oikos_dev}@postgres:5432/oikos?sslmode=disable
       OIKOS_DEBUG: "true"
       OIKOS_SCHEDULER_INTERVAL: "30s"
+      OIKOS_SSH_KEY_PATH: /etc/oikos/ssh_key
+    volumes:
+      - ${OIKOS_SSH_KEY_PATH:-~/.ssh/id_ed25519}:/etc/oikos/ssh_key:ro
+    cap_add:
+      - NET_RAW
     command: ["scheduler"]
     stop_signal: SIGTERM
     stop_grace_period: 30s