docs: import: update list of supported options for --change

Update the list of accepted "change" commands to match what's accepted
by the daemon. This list is the same for "docker commit" and "docker import",
which is defined by the [`validCommitCommands`] variable.

[`validCommitCommands`]: https://github.com/moby/moby/blob/v28.1.1/builder/dockerfile/builder.go#L30-L42

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

Author: thaJeztah

docs/reference/commandline/image_import.md

@@ -74,7 +74,10 @@ instructions are supported:
 - [`ENTRYPOINT`](https://docs.docker.com/reference/dockerfile/#entrypoint)
 - [`ENV`](https://docs.docker.com/reference/dockerfile/#env)
 - [`EXPOSE`](https://docs.docker.com/reference/dockerfile/#expose)
+- [`HEALTHCHECK`](https://docs.docker.com/reference/dockerfile/#healthcheck)
+- [`LABEL`](https://docs.docker.com/reference/dockerfile/#label)
 - [`ONBUILD`](https://docs.docker.com/reference/dockerfile/#onbuild)
+- [`STOPSIGNAL`](https://docs.docker.com/reference/dockerfile/#stopsignal)
 - [`USER`](https://docs.docker.com/reference/dockerfile/#user)
 - [`VOLUME`](https://docs.docker.com/reference/dockerfile/#volume)
 - [`WORKDIR`](https://docs.docker.com/reference/dockerfile/#workdir)