Move Cassandra env variable setup to protected method

With testcontainers#2830 we added some environment variable setup to the
constructor in CassandraContainer. This causes problems in
scenarios where users customize the environment themselves
or customize the values in cassandra.yaml, so move this init
to a protected method to facilitate overriding.

modules/cassandra/src/main/java/org/testcontainers/containers/CassandraContainer.java

@@ -68,13 +68,6 @@ public CassandraContainer(DockerImageName dockerImageName) {
 
         addExposedPort(CQL_PORT);
         this.enableJmxReporting = false;
-
-        withEnv("CASSANDRA_SNITCH", "GossipingPropertyFileSnitch");
-        withEnv("JVM_OPTS", "-Dcassandra.skip_wait_for_gossip_to_settle=0 -Dcassandra.initial_token=0");
-        withEnv("HEAP_NEWSIZE", "128M");
-        withEnv("MAX_HEAP_SIZE", "1024M");
-        withEnv("CASSANDRA_ENDPOINT_SNITCH", "GossipingPropertyFileSnitch");
-        withEnv("CASSANDRA_DC", DEFAULT_LOCAL_DATACENTER);
     }
 
     @Override
@@ -131,6 +124,19 @@ protected void optionallyMapResourceParameterAsVolume(String pathNameInContainer
             .ifPresent(mountableFile -> withCopyFileToContainer(mountableFile, pathNameInContainer));
     }
 
+    /**
+     * Set up environment variables to allow Cassandra to start up. You may choose to override this method for example
+     * if you want to set your own variables to run multi-node clusters.
+     */
+    protected void initEnv() {
+        withEnv("CASSANDRA_SNITCH", "GossipingPropertyFileSnitch");
+        withEnv("JVM_OPTS", "-Dcassandra.skip_wait_for_gossip_to_settle=0 -Dcassandra.initial_token=0");
+        withEnv("HEAP_NEWSIZE", "128M");
+        withEnv("MAX_HEAP_SIZE", "1024M");
+        withEnv("CASSANDRA_ENDPOINT_SNITCH", "GossipingPropertyFileSnitch");
+        withEnv("CASSANDRA_DC", getLocalDatacenter());
+    }
+
     /**
      * Initialize Cassandra with the custom overridden Cassandra configuration
      * <p>