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/test/java/google/registry/sql/flyway/SchemaTest.java

@@ -79,7 +79,7 @@ class SchemaTest {
    */
   @Container
   private final PostgreSQLContainer<?> sqlContainer =
-      new PostgreSQLContainer<>(NomulusPostgreSql.getDockerTag())
+      new PostgreSQLContainer<>(NomulusPostgreSql.getDockerImageName())
           .withClasspathResourceMapping(
               MOUNTED_RESOURCE_PATH, CONTAINER_MOUNT_POINT, BindMode.READ_WRITE);