commands: add debug as persistent flag

Allows using `--debug` to enable debug logging under
any subcommand. Currently it needed to be set as
`docker --debug buildx` meaning only way to enable debug
in standalone mode was to set env variable instead and
updating existing commands to add `--debug` was cumbersome.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

docs/reference/buildx_version.md

@@ -7,6 +7,12 @@ docker buildx version
 <!---MARKER_GEN_START-->
 Show buildx version information
 
+### Options
+
+| Name            | Type   | Default | Description          |
+|:----------------|:-------|:--------|:---------------------|
+| `-D`, `--debug` | `bool` |         | Enable debug logging |
+
 
 <!---MARKER_GEN_END-->