Billing docs improvement + build opt

ci/scripts/tgsettle/build-image.sh

@@ -90,6 +90,9 @@ if [ ! -f "${BUILD_CONTEXT}/${TGSETTLE_GATEWAY_DOCKERFILE}" ]; then
   exit 68
 fi
 
+KANIKO_CACHE_REPO="${KANIKO_CACHE_REPO:-${REGISTRY_URL}/kaniko-cache}"
+KANIKO_CACHE_TTL="${KANIKO_CACHE_TTL:-336h}"
+
 /kaniko/executor \
   --context "${BUILD_CONTEXT}" \
   --dockerfile "${TGSETTLE_GATEWAY_DOCKERFILE}" \
@@ -99,4 +102,8 @@ fi
   --build-arg BUILD_BRANCH="${BUILD_BRANCH}" \
   --build-arg BUILD_DATE="${BUILD_DATE}" \
   --build-arg BUILD_USER="${BUILD_USER}" \
-  --single-snapshot
+  --cache=true \
+  --cache-copy-layers \
+  --cache-run-layers \
+  --cache-repo "${KANIKO_CACHE_REPO}" \
+  --cache-ttl "${KANIKO_CACHE_TTL}"