Invalidate build cache less often

ethd

@@ -2026,14 +2026,74 @@ __nag_os_update() {
 }
 
 
+__docker_df_to_bytes() {
+  local awk_exe
+  local num
+  local unit
+
+  if [[ "$OSTYPE" = "darwin"* ]]; then
+    awk_exe="gawk"  # Built-in awk is BSD and may behave in ways I do not code for
+  else
+    awk_exe="awk"
+  fi
+
+  if [[ "$1" =~ ^([0-9.]+)([kMGTP]?B)$ ]]; then
+    num="${BASH_REMATCH[1]}"
+    unit="${BASH_REMATCH[2]}"
+  else
+    echo "Docker build cache size parsing failed. This may be a bug"
+    return
+  fi
+
+  ${awk_exe} -v num="${num}" -v unit="${unit}" '
+    # docker system df reports GB, not GiB
+    function pow1000(n, i) { r=1; for (i=0;i<n;i++) r*=1000; return r }
+
+    BEGIN {
+      if (unit == "B")      bytes = num
+      else if (unit == "kB") bytes = num * pow1000(1)
+      else if (unit == "MB") bytes = num * pow1000(2)
+      else if (unit == "GB") bytes = num * pow1000(3)
+      else if (unit == "TB") bytes = num * pow1000(4)
+      else if (unit == "PB") bytes = num * pow1000(5)  # OK yes I am being silly
+      else bytes = 0  # In case bash format parsing is incorrect
+
+      printf "%.0f\n", bytes
+    }
+  '
+}
+
+
+__prune_docker_cache() {
+  local val
+  local bytes
+  local threshold
+
+  val=$(__dodocker system df --format '{{if eq .Type "Build Cache"}}{{.Reclaimable}}{{end}}' \
+      | sed '/^$/d' | awk '{print $1}' || true)
+  if [[ ! "${val}" =~ ^[0-9.]+[kMGTP]?B$ ]]; then
+    echo "Docker build cache size query failed. This may be a bug."
+    return
+  fi
+  bytes=$(__docker_df_to_bytes "${val}")
+
+  threshold=$((50 * 1000 * 1000 * 1000 ))  # 50GB
+  if [[ "$bytes" =~ ^[0-9]+$ && "${bytes}" -gt "${threshold}" ]]; then
+    echo "Pruning Docker build cache, >50GB reclaimable."
+    __dodocker system prune --force
+  fi
+}
+
+
 __pull_and_build() {
   echo "Building local client images"
-  __dodocker system prune --force
+  __prune_docker_cache
   __docompose --profile tools pull
   __source_build
   __docompose --profile tools build --pull
 }
 
+
 # Run optional pre-update hook script for custom actions before update/config.
 # Users can create pre-ethd-update.sh to run validation or preparation steps.
 __run_pre_update_script() {