Live capture: pin default docker image

Tracee may introduce changes that break the dissector plugins or the live capture plugin.
Pinning the default docker image to the latest stable version will prevent
any problems with the default configuration.

extcap/tracee-capture.py

@@ -49,7 +49,7 @@
 TRACEE_OPTIONS_GROUP = 'Tracee options'
 
 DEFAULT_CAPTURE_TYPE = 'local'
-DEFAULT_TRACEE_IMAGE = 'aquasec/tracee:latest'
+DEFAULT_TRACEE_IMAGE = 'aquasec/tracee:0.21.0'
 DEFAULT_DOCKER_OPTIONS = '--pid=host --cgroupns=host --privileged -v /etc/os-release:/etc/os-release-host:ro -v /var/run:/var/run:ro -v /sys/fs/cgroup:/sys/fs/cgroup -v /var/run/docker.sock:/var/run/docker.sock'
 DEFAULT_CONTAINER_NAME = 'traceeshark'
 DEFAULT_LOGFILE = os.path.join(TMP_DIR, 'tracee_logs.log')