all: prepare 0.29.6 security updates

Updates #3579

Dockerfile.tailscale-HEAD

@@ -4,7 +4,7 @@
 # This Dockerfile is more or less lifted from tailscale/tailscale
 # to ensure a similar build process when testing the HEAD of tailscale.
 
-FROM golang:1.27.2-alpine AS build-env
+FROM golang:1.27.3-alpine AS build-env
 
 # The builder pin is a floor, not a ceiling: this stage compiles a tailscale
 # tree cloned from an unpinned branch, which can raise its go directive at any