Update enviroment variable to expand symbolic referrence to Docker Swarm secret with the actual value.
By convention, enviroment values with value "secret:<name>" will have their value replaced
by the content of file /run/secrets/<name>. The path to secrets may be overriden by setting
$SECRETS_PATH
The intention is to run this as a wrapper for another command which already expects the values to be present in enviroment variable. For example:
secret2value confd -onetime -backend env