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# Type of request: This is a bug report

# Detailed description
When running a java lambda in localstack with `LAMBDA_EXECUTOR=docker` and  `LAMBDA_JAVA_OPTS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005"` (replaced `_debug_port_` with `5005`), the port is not opened in the lambda execution docker container.

Looking at `Util.get_java_opts` in `localstack/services/awslambda/lambda_executors.py`, `debug_java_port` is not set in case `_debug_port_` is set

```
class Util:
    debug_java_port = False

    @classmethod
    def get_java_opts(cls):
        opts = config.LAMBDA_JAVA_OPTS or ''
        if '_debug_port_' in opts:
            if not cls.debug_java_port:
                cls.debug_java_port = get_free_tcp_port() ### <<< debug_java_port is only set here
            opts = opts.replace('_debug_port_', ('%s' % cls.debug_java_port))
        ### In other case, `cls.debug_java_port` is not set.
        return opts
```
## Expected behavior

`debug_java_port` should be set to the configured port

## Actual behavior

`debug_java_port` is empty

# Steps to reproduce
docker-compose file
```
version: '2.1'
services:
  localstack:
    container_name: localstack
    image: localstack/localstack
    privileged: true
    ports:
      - "4567-4582:4567-4582"
      - "9070:9070"
    environment:
      SERVICES: sqs,lambda
      DOCKER_HOST: unix:///var/run/docker.sock
      PORT_WEB_UI: 9070
      LAMBDA_EXECUTOR: docker
      LAMBDA_JAVA_OPTS: "-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005"
      DEBUG: 1
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /tmp/localstack:/tmp/localstack
```

## Command used to start LocalStack

`docker-compose up`


environment['AWS_LAMBDA_FUNCTION_VERSION'] = context.function_version
            environment['AWS_LAMBDA_FUNCTION_INVOKED_ARN'] = context.invoked_function_arn
        
        command = ''
        
        if is_java_lambda(runtime):
            java_opts = Util.get_java_opts()
            stdin = None
            
            target_file = os.path.join(lambda_cwd, os.path.basename(LAMBDA_EXECUTOR_JAR))
            if not os.path.exists(target_file):
                cp_r(LAMBDA_EXECUTOR_JAR, target_file)
            
            taskdir = '/var/task'
            save_file(os.path.join(lambda_cwd, LAMBDA_EVENT_FILE), event_body)
            classpath = Util.get_java_classpath(target_file)
            command = ("bash -c 'cd %s; java %s -cp \"%s\" \"%s\" \"%s\" \"%s\"'" %
                (taskdir, java_opts, classpath, LAMBDA_EXECUTOR_CLASS, handler, LAMBDA_EVENT_FILE))
        
        if is_nodejs_runtime(runtime):
            environment['NODE_TLS_REJECT_UNAUTHORIZED'] = '0'
        
        cmd = self.prepare_execution(func_arn, environment, runtime, command, handler, lambda_cwd)
        
        LOG.info('Running lambda cmd: %s' % cmd)
        result = self.run_lambda_executor(cmd, stdin, env_vars=environment, func_details=func_details)
        return result
class LambdaExecutorReuseContainers(LambdaExecutorContainers):
    """ Executor class for executing Lambda functions in re-usable Docker containers """
    def __init__(self):