corefile.exp: XFAIL warning-free test when testing on docker

When testing on docker using the AUFS storage driver, loading a core
file will often print a number of warnings.  Here's an example (with
the pathname shortened somewhat):

warning: Can't open file /var/lib/docker/aufs/diff/d07..e21/lib/x86_64-linux-gnu/libc-2.27.so during file-backed mapping note processing

The "warning-free" test in gdb.base/corefile.exp will fail if any
warnings are printed, but this particular warning is unavoidable when
running in the docker environment.  Fortunately, the path mentions
both "docker" and "aufs", making it easy to XFAIL this case.

gdb/testsuite/ChangeLog:

	* gdb.base/corefile.exp (warning-free): XFAIL test when running
	on docker w/ AUFS storage driver.