Add an end-to-end test for syslog redirection

1. Make the path to syslog used by SyslogRedirectingContainerDecorator
   configurable via the SYSLOG_REDIRECTOR job environment variable.

2. With an image that has a prespecified ENTRYPOINT and CMD, test that
   redirection actually works is sending stdout to the specified endpoint,
   while respecting the command specified in the Dockerfile.
Consider extracting the log reader into a separate function.