Make a best effort attempt to support multiple CPU architectures (#2672)

I obtained access to an IBM s390x VM so I thought I'd see how multi-arch
Nomulus is.

Our main application is in Java so it is already multi-arch, but several
tests use docker images that are by default x64. Luckily postgres has an
s390x port, but selenium does not. So I had to disable Screenshot tests
when the arch is not amd64.

db/src/main/java/google/registry/persistence/NomulusPostgreSql.java

@@ -13,15 +13,27 @@
 // limitations under the License.
 package google.registry.persistence;
 
+import org.testcontainers.utility.DockerImageName;
+
 /** Information about Nomulus' Cloud SQL PostgreSql instance. */
 public class NomulusPostgreSql {
 
+  /** Get the current system architecture, used to deduce the docker image name. */
+  private static final String ARCH = System.getProperty("os.arch");
+
   /** The current PostgreSql version in Cloud SQL. */
   // TODO(weiminyu): setup periodic checks to detect version changes in Cloud SQL.
   private static final String TARGET_VERSION = "11.21-alpine";
 
-  /** Returns the docker image tag of the targeted Postgresql server version. */
-  public static String getDockerTag() {
-    return "postgres:" + TARGET_VERSION;
+  /**
+   * Returns the docker image of the targeted Postgresql server version.
+   *
+   * <p>If the architecture is not amd64, the image will be prefixed with the architecture name.
+   *
+   * @see <a href="https://hub.docker.com/_/postgres">Postgres Docker Hub</a>
+   */
+  public static DockerImageName getDockerImageName() {
+    String image = (ARCH.equals("amd64") ? "" : ARCH + "/") + "postgres:" + TARGET_VERSION;
+    return DockerImageName.parse(image).asCompatibleSubstituteFor("postgres");
   }
 }