Previously, when the Spring Boot CLI ran the Java command line for an
application it did a straight invocation which suppressed propagation of
signals to the JVM in certain circumstances (e.g within Docker containers).

This change prepends the command with exec which causes the Java process to
replace the script process in a shell ensuring that signals are propagated to
the Java process without suppression.

See gh-17976
Signed-off-by: Ben Hale <bhale@pivotal.io>