Provide the `/proc/config.gz` file in guest kernels that allow the guest
to determine the kernel configuration used to build the running kernel.
Note that since `gunzip` expects to rename the gzip'ed file it operates
on, to use this feature you need to run something like the following in
the container environment:
```bash
# cat /proc/config.gz|gunzip -c
```
Fixes: #3445.
Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>