travis: save docker images on local cache

The recently introduced rate limits on Docker Hub are constantly being
reached by every TravisCI because each individual job that creates a
MySQL container, tries to pull the base image from Docker Hub. With a
not-so-small test matrix, this issue surfaces pretty fast.

This patch introduces a local cache to save docker images and be able to
re-use them on different jobs and avoid constantly pulling them from
Docker Hub.

.travis.yml

@@ -1,3 +1,7 @@
+# This file was modified by Oracle on June 14, 2021.
+# The changes involve the introduction of a local cache for Docker images.
+# Modifications copyright (c) 2021, Oracle and/or its affiliates.
+
 sudo: required
 dist: trusty
 
@@ -9,9 +13,19 @@ language: node_js
 cache:
   yarn: true
   directories:
+    - docker_images
     - node_modules
     - $HOME/.yarn-cache
 
+before_cache:
+  # save all docker images to a local cache in order to avoid the rate limit
+  # on Docker Hub
+  - docker save -o docker_images/images.tar $(docker images -a -q)
+
+before_install:
+  # load docker images from the local cache
+  - docker load -i docker_images/images.tar || true
+
 # Node.js version:
 # we test only maintained LTS versions
 # and lastest dev version