Dockerfile,build_docker.sh: add a note on how to build local images (#16471)

Updates#cleanup

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

Dockerfile

@@ -7,6 +7,15 @@
 # Tailscale images are currently built using https://github.com/tailscale/mkctr,
 # and the build script can be found in ./build_docker.sh.
 #
+# If you want to build local images for testing, you can use make.
+#
+# To build a Tailscale image and push to the local docker registry:
+#
+#   $ REPO=local/tailscale TAGS=v0.0.1 PLATFORM=local  make publishdevimage
+#
+# To build a Tailscale image and push to a remote docker registry:
+#
+#   $ REPO=<your-registry>/<your-repo>/tailscale TAGS=v0.0.1  make publishdevimage
 #
 # This Dockerfile includes all the tailscale binaries.
 #

build_docker.sh

@@ -6,6 +6,16 @@
 # hash of this repository as produced by ./cmd/mkversion.
 # This is the image build mechanim used to build the official Tailscale
 # container images.
+#
+# If you want to build local images for testing, you can use make, which provides few convenience wrappers around this script.
+#
+# To build a Tailscale image and push to the local docker registry:
+
+#   $ REPO=local/tailscale TAGS=v0.0.1 PLATFORM=local  make publishdevimage
+#
+# To build a Tailscale image and push to a remote docker registry:
+#
+#   $ REPO=<your-registry>/<your-repo>/tailscale TAGS=v0.0.1  make publishdevimage
 
 set -eu