Dockerfile: add big warning banner
Updates
# Use of this source code is governed by a BSD-style
# license that can be found in the LICENSE file.
+############################################################################
+#
+# WARNING: Tailscale is not yet officially supported in Docker,
+# Kubernetes, etc.
+#
+# It might work, but we don't regularly test it, and it's not as polished as
+# our currently supported platforms. This is provided for people who know
+# how Tailscale works and what they're doing.
+#
+# Our tracking bug for officially support container use cases is:
+# https://github.com/tailscale/tailscale/issues/504
+#
+# Also, see the various bugs tagged "containers":
+# https://github.com/tailscale/tailscale/labels/containers
+#
+############################################################################
+
# This Dockerfile includes all the tailscale binaries.
#
# To build the Dockerfile:
