Update Alpine's Ubuntu example to 18.04

alpine/content.md

@@ -19,14 +19,14 @@ ENTRYPOINT ["mysql"]
 This example has a virtual image size of only 36.8MB. Compare that to our good friend Ubuntu:
 
 ```dockerfile
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update \
     && apt-get install -y --no-install-recommends mysql-client \
     && rm -rf /var/lib/apt/lists/*
 ENTRYPOINT ["mysql"]
 ```
 
-This yields us a virtual image size of about 179MB image.
+This yields us a virtual image size of about 145MB image.
 
 ## Documentation