Add a paragraph about how to force a stack trace dump to the daemon log.
This feature was added in Docker 1.9 I believe, but documentation was
never added.
Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
Upstream-commit: ae466aafcbd3e000ff9ec2e8bd354fc3cc3729e5
Component: engine