update flag-description for --cgroup-parent

This attempts to make it clearer that the --cgroup-parent option is only used
for the containers used during build. Instead of mentioning "build container",
I opted for using "RUN instructions" (to match the --network description),
although this may not be ideal (as it assumes the "Dockerfile" front-end, which
of course may not be the case).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

commands/build.go

@@ -464,7 +464,7 @@ func buildCmd(dockerCli command.Cli, rootOpts *rootOptions) *cobra.Command {
 
 	flags.StringArrayVar(&options.cacheTo, "cache-to", []string{}, `Cache export destinations (e.g., "user/app:cache", "type=local,dest=path/to/dir")`)
 
-	flags.StringVar(&options.cgroupParent, "cgroup-parent", "", "Optional parent cgroup for the container")
+	flags.StringVar(&options.cgroupParent, "cgroup-parent", "", `Set the parent cgroup for the "RUN" instructions during build`)
 	flags.SetAnnotation("cgroup-parent", annotation.ExternalURL, []string{"https://docs.docker.com/engine/reference/commandline/build/#cgroup-parent"})
 
 	flags.StringArrayVar(&options.contexts, "build-context", []string{}, "Additional build contexts (e.g., name=path)")

docs/reference/buildx_build.md

@@ -23,7 +23,7 @@ Start a build
 | [`--builder`](#builder)                                                                                                                                | `string`      |           | Override the configured builder instance                                                            |
 | [`--cache-from`](#cache-from)                                                                                                                          | `stringArray` |           | External cache sources (e.g., `user/app:cache`, `type=local,src=path/to/dir`)                       |
 | [`--cache-to`](#cache-to)                                                                                                                              | `stringArray` |           | Cache export destinations (e.g., `user/app:cache`, `type=local,dest=path/to/dir`)                   |
-| [`--cgroup-parent`](https://docs.docker.com/engine/reference/commandline/build/#cgroup-parent)                                                         | `string`      |           | Optional parent cgroup for the container                                                            |
+| [`--cgroup-parent`](https://docs.docker.com/engine/reference/commandline/build/#cgroup-parent)                                                         | `string`      |           | Set the parent cgroup for the `RUN` instructions during build                                       |
 | `--detach`                                                                                                                                             |               |           | Detach buildx server (supported only on linux)                                                      |
 | [`-f`](https://docs.docker.com/engine/reference/commandline/build/#file), [`--file`](https://docs.docker.com/engine/reference/commandline/build/#file) | `string`      |           | Name of the Dockerfile (default: `PATH/Dockerfile`)                                                 |
 | `--iidfile`                                                                                                                                            | `string`      |           | Write the image ID to the file                                                                      |