Hi,

Since environment variables are global, I think they should be prefixed by `LOCALSTACK_` in the docker image. For example `SERVICES` environment variable might conflict at some point with something else, so naming it `LOCALSTACK_SERVICES` would be nicer.

This could be optional to avoid breaking changes. Localstack would look for `LOCALSTACK_SERVICES` first, and if it's not defined would fallback to `SERVICES` just as today.

As far as I know, most (maybe all) official docker images do this. I think it's a good practice. See:
- https://hub.docker.com/_/postgres
- https://hub.docker.com/_/nginx
- https://hub.docker.com/_/mysql

Thanks.
LAMBDA_REMOTE_DOCKER = os.environ.get('LAMBDA_REMOTE_DOCKER', '').lower().strip() in ['true', '1']
LAMBDA_DOCKER_NETWORK = os.environ.get('LAMBDA_DOCKER_NETWORK', '').strip()
TMP_FOLDER = os.path.join(tempfile.gettempdir(), 'localstack')
if TMP_FOLDER.startswith('/var/folders/') and os.path.exists('/private%s' % TMP_FOLDER):
    TMP_FOLDER = '/private%s' % TMP_FOLDER
HOST_TMP_FOLDER = os.environ.get('HOST_TMP_FOLDER', TMP_FOLDER)
DATA_DIR = os.environ.get('DATA_DIR', '').strip()
USE_SSL = os.environ.get('USE_SSL', '').strip() not in ('0', 'false', '')
DEFAULT_ENCODING = 'utf-8'
DOCKER_SOCK = os.environ.get('DOCKER_SOCK', '').strip() or '/var/run/docker.sock'
PORT_WEB_UI = int(os.environ.get('PORT_WEB_UI', '').strip() or DEFAULT_PORT_WEB_UI)
LAMBDA_EXECUTOR = os.environ.get('LAMBDA_EXECUTOR', '').strip()
if not LAMBDA_EXECUTOR:
    LAMBDA_EXECUTOR = 'local'
    try:
        if 'Linux' in subprocess.check_output('uname -a'):
            LAMBDA_EXECUTOR = 'docker'
    except Exception:
        pass
CONFIG_ENV_VARS = ['SERVICES', 'HOSTNAME', 'HOSTNAME_EXTERNAL', 'LOCALSTACK_HOSTNAME',
    'LAMBDA_EXECUTOR', 'LAMBDA_REMOTE_DOCKER', 'LAMBDA_DOCKER_NETWORK', 'USE_SSL', 'LICENSE_KEY', 'DEBUG',
    'KINESIS_ERROR_PROBABILITY', 'DYNAMODB_ERROR_PROBABILITY', 'PORT_WEB_UI', 'START_WEB']
for key, value in iteritems(DEFAULT_SERVICE_PORTS):
    backend_override_var = '%s_BACKEND' % key.upper().replace('-', '_')
    if os.environ.get(backend_override_var):
        CONFIG_ENV_VARS.append(backend_override_var)
    port_external_override_var = '%s_PORT_EXTERNAL' % key.upper().replace('-', '_')
    if os.environ.get(port_external_override_var):
        CONFIG_ENV_VARS.append(port_external_override_var)
def in_docker():