build_docker.sh: update script comment (tailscale#12970)

It is no longer correct to state that we don't support running Tailscale in containers or on Kubernetes.

Updates tailscale#12842

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

build_docker.sh

@@ -1,21 +1,11 @@
 #!/usr/bin/env sh
-
-#
-# Runs `go build` with flags configured for docker distribution. All
-# it does differently from `go build` is burn git commit and version
-# information into the binaries inside docker, so that we can track down user
-# issues.
-#
-############################################################################
-#
-# WARNING: Tailscale is not yet officially supported in container
-# environments, such as Docker and Kubernetes. Though it should work, we
-# don't regularly test it, and we know there are some feature limitations.
-#
-# See current bugs tagged "containers":
-#    https://github.com/tailscale/tailscale/labels/containers
 #
-############################################################################
+# This script builds Tailscale container images using
+# github.com/tailscale/mkctr.
+# By default the images will be tagged with the current version and git
+# hash of this repository as produced by ./cmd/mkversion.
+# This is the image build mechanim used to build the official Tailscale
+# container images.
 
 set -eu