util/docker/Makefile: Create Documentation/_build for docker-build-docs

If the host directory of a bind mount does not exist, Docker will create
it. However, the newly created directory will be owned by root due to
the Docker service running within a root context. The docker command in
the recipe for docker-build-docs binds Documentation/_build to /data-out
within the container, so if it doesn't already exist, the documentation
builder will be unable to copy the HTML output into /data-out since it
runs with the same UID and GID as the host user.

By creating, if necessary, the _build directory before the `docker run`
command, there should always be an existing directory owned by the host
user for docker to bind /data-out to (ignoring the case of an existing
_build directory the current user does not have permission to write to),
avoiding the issue where it cannot write the output.

TEST: make -C util/docker docker-build-docs completes without issues
with and without an existing Documentation/_build directory

Change-Id: I6be9bc1fdca48f4d924f5c07cc261189ab6862fd
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81127
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

util/docker/Makefile

@@ -177,6 +177,7 @@ docker-jenkins-attach: test-docker
 
 docker-build-docs: test-docker
 docker-build-docs:
+	mkdir -p $(top)/Documentation/_build
 	$(DOCKER) run -it --rm \
 		--user $(UID):$(GID) \
 		-v "$(top)/:/data-in/:ro" \