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.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit e1148e2c70f9f94d3dd1c7e41e20570bb4f6c723)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 521a636a86ccb8af19e534235160e641ef04c26c
Component: cli

Author: sargun

components/cli/docs/reference/commandline/dockerd.md

@@ -127,6 +127,8 @@ 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