fix: Use exact version number for postgress

[teogeb]

Before this PR the postgres service didn't specify any version number. I.e. used the latest version . When v18 was released on 2025-09-25, we started to get this error in network monorepo's GitHub action logs:

Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: error mounting "/var/lib/docker/volumes/streamr-docker-dev_data-postgres-fastchain/_data" to rootfs at "/var/lib/postgresql/data": change mount propagation through procfd: open o_path procfd: open /var/lib/docker/overlay2/1064c70ea0d33db456a30333bef5a49e1cbf1a98b4d097def04149a63bca145f/merged/var/lib/postgresql/data: no such file or directory: unknown
streamr-docker-dev: error while running command: "docker compose --ansi never -f docker-compose.yml  -f docker-compose-ci.yml up  -d --remove-orphans cassandra init-keyspace dev-chain-fast deploy-network-subgraphs-fastchain broker-node-storage-1 entry-point "

Most likely there is some incompatibility with v18. As v17 worked ok, we use that version now in docker-compose.yml.