Pin scylla db docker container (#2939)

* Pin ScyllaDB version in GitHub workflow.

* Pin ScyllaDB version in docker compose.

Author: christos-h

.github/workflows/scylladb.yml

@@ -53,7 +53,7 @@ jobs:
         cargo build --locked --features scylladb
     - name: Setup local ScyllaDB instance
       run: |
-        docker run --name my_scylla_container -d -p 9042:9042 scylladb/scylla
+        docker run --name my_scylla_container -d -p 9042:9042 scylladb/scylla:6.1
     - name: Run ScyllaDB tests
       run: |
         RUST_LOG=linera=info cargo test --locked --features scylladb -- scylla --nocapture

docker/docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   scylla:
-    image: scylladb/scylla:latest
+    image: scylladb/scylla:6.1
     container_name: scylla
     volumes:
       - linera-scylla-data:/var/lib/scylla