Add MOBY_DISABLE_PIGZ to dockerd environment variable docs
This reflects a Moby change to add documentation around
disabling a new feature -- to use pgzip to decompress
layers, rather than the built-in go gzip.
@@ -130,6 +130,8 @@ by the `dockerd` command line:
unsuitable for Docker.
* `DOCKER_RAMDISK` If set this will disable 'pivot_root'.
* `DOCKER_TMPDIR` Location for temporary Docker files.
+* `MOBY_DISABLE_PIGZ` Do not use [`unpigz`](https://linux.die.net/man/1/pigz) to
+ decompress layers in parallel when pulling images, even if it is installed.
## Examples
