test: specify docker API version 1.41 for structure test (#2861)

This test was failing at head (#2856) with error "Error response from
daemon: client version 1.52 is too new. Maximum supported API version is
1.41" since Dockerfile is pulling 29.0.0. It's likely that the new major
version removed some backward-compatibility logic that allowed the
client to auto downgrade its API version.

The Docker client inside librarian image is too new (1.52) for the
Docker server environment (1.41) provided by Cloud Build, pin the
version for the test.
This change is tested with `gcloud --project=<my-project> builds submit
. --config=cloudbuild-test.yaml` , see success
[log](https://screenshot.googleplex.com/C9yXpqpa4sb9Yax).

Fixes #2860

Author: zhumin8

container-structure-test.yaml

@@ -16,11 +16,14 @@ commandTests:
   - name: "librarian version"
     command: ["/app/librarian", "version"]
     expectedOutput: ["\\d+\\.\\d+\\.\\d+"]
-    # temporarily disabled, tracked in
-    # https://github.com/googleapis/librarian/issues/2860
-    #  - name: "docker"
-    #    command: ["docker", "version"]
-    #    expectedOutput: ["Docker Engine", "Version:"]
+  - name: "docker"
+    command: ["docker", "version"]
+    expectedOutput: ["Docker Engine", "Version:"]
+    envVars:
+      # Pin the Docker API version to match the one supported by the
+      # Docker server in the Cloud Build environment.
+      - key: "DOCKER_API_VERSION"
+        value: "1.41"
   - name: "gcloud"
     command: ["gcloud", "--version"]
     expectedOutput: ["Google Cloud SDK"]