I'm trying to start localstack in a containerized Jenkins build agent for unit testing. In other words, trying to start the localstack docker container from within a docker container. We do this all the time with other images, using the method of mounting `/var/run/docker.sock` in the "parent" container, but localstack will not start.

I get this error:

```
2019-11-08T18:53:53:ERROR:localstack.services.generic_proxy: Exception running proxy on port 8081: [Errno 13] Permission denied: '/tmp/localstack/server.test.pem' Traceback (most recent call last):
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 384, in run_cmd
    combined_file, cert_file_name, key_file_name = GenericProxy.create_ssl_cert(serial_number=self.port)
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 401, in create_ssl_cert
    return generate_ssl_cert(SERVER_CERT_PEM_FILE, serial_number=serial_number)
  File "/opt/code/localstack/localstack/utils/common.py", line 828, in generate_ssl_cert
    save_file(target_file, file_content)
  File "/opt/code/localstack/localstack/utils/common.py", line 642, in save_file
    with open(file, mode) as f:
PermissionError: [Errno 13] Permission denied: '/tmp/localstack/server.test.pem'
```

Port 8081 is not already in use, and `/tmp/localstack` is writable, so I don't understand why it's getting permission denied.
try:
        if not os.path.exists('cat /etc/issue'):
            return False
        out = to_str(subprocess.check_output('cat /etc/issue', shell=True))
        return 'Alpine' in out
    except subprocess.CalledProcessError:
        return False
def short_uid():
    return str(uuid.uuid4())[0:8]
def json_safe(item):
    """ return a copy of the given object (e.g., dict) that is safe for JSON dumping """
    try:
        return json.loads(json.dumps(item, cls=CustomEncoder))
    except Exception:
        item = fix_json_keys(item)
        return json.loads(json.dumps(item, cls=CustomEncoder))
def fix_json_keys(item):
    """ make sure the keys of a JSON are strings (not binary type or other) """
    item_copy = item
    if isinstance(item, list):
        item_copy = []
        for i in item:
            item_copy.append(fix_json_keys(i))
    if isinstance(item, dict):
        item_copy = {}
        for k, v in item.items():
            item_copy[to_str(k)] = fix_json_keys(v)
    return item_copy
def save_file(file, content, append=False):