Add script for docker events/metrics and support running TA outside of Splunk

* Add docker.sh and docker_metric.sh for collecting docker events/metrics
* Add helper script to extra/ to run the TA commands on systems without
  a Splunk forwarder. The commands can be sent to a syslog server.
  This script is useful for systems with small or read-only filesystems that
  cannot support a Universal Forwarder.
* Add syslog_inputs_nix_ta app to extra/ for ingesting the data from syslog

default/inputs.conf

@@ -4,6 +4,12 @@
 ##
 ##
 
+[script://./bin/docker_metric.sh]
+sourcetype = docker_metric
+source = docker
+interval = 60
+disabled = 1
+
 [script://./bin/vmstat_metric.sh]
 sourcetype = vmstat_metric
 source = vmstat
@@ -44,6 +50,12 @@ disabled = 1
 ############### Event Inputs ###################
 ################################################
 
+[script://./bin/docker.sh]
+interval = 60
+sourcetype = docker
+source = docker
+disabled = 1
+
 [script://./bin/vmstat.sh]
 interval = 60
 sourcetype = vmstat