<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# 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`


try:
        return event['Records'][0][key]
    except KeyError:
        return None
def is_java_lambda(lambda_details):
    runtime = getattr(lambda_details, 'runtime', lambda_details)
    return runtime in [LAMBDA_RUNTIME_JAVA8, LAMBDA_RUNTIME_JAVA11]
def is_nodejs_runtime(lambda_details):
    runtime = getattr(lambda_details, 'runtime', lambda_details)
    return runtime.startswith('nodejs')
def _store_logs(func_details, log_output, invocation_time=None, container_id=None):
    log_group_name = '/aws/lambda/%s' % func_details.name()
    container_id = container_id or short_uid()
    invocation_time = invocation_time or int(time.time() * 1000)
    invocation_time_secs = int(invocation_time / 1000)
    time_str = time.strftime('%Y/%m/%d', time.gmtime(invocation_time_secs))
    log_stream_name = '%s/[LATEST]%s' % (time_str, container_id)
    return store_cloudwatch_logs(log_group_name, log_stream_name, log_output, invocation_time)
class LambdaExecutor(object):
    """ Base class for Lambda executors. Subclasses must overwrite the _execute method """
    def __init__(self):