Makefile: update Docker image every three months

Update Docker image every three months to force periodic rebuilds.

Fixes tailscale/corp#26816
Signed-off-by: kari-ts <kari@tailscale.com>

Author: kari-ts

Makefile

@@ -10,7 +10,19 @@
 # with this name, it will be used.
 #
 # The convention here is tailscale-android-build-amd64-<date>
-DOCKER_IMAGE=tailscale-android-build-amd64-191124
+#
+# Compute a quarterly version string.
+year=$(date +%y)
+month=$(date +%m)
+# Convert month to an integer.
+month_int=$(printf "%d" "$month")
+# Calculate the quarter's start month:
+#   Quarter = floor((month-1)/3), then start = (quarter*3)+1.
+quarter_start=$(( ((month_int - 1) / 3) * 3 + 1 ))
+formatted_quarter=$(printf "%02d" "$quarter_start")
+
+# Set the Docker image name. This will update every 3 months.
+DOCKER_IMAGE="tailscale-android-build-amd64-${year}${formatted_quarter}"
 export TS_USE_TOOLCHAIN=1
 
 DEBUG_APK=tailscale-debug.apk