Update docker image for Java

[tom-tan]

This request updates deprecated Java docker image java:7-jdk to openjdk:9.0.1-11-slim.

I used openjdk:9.0.1-11-slim as the latest image.

• I used slim family for smaller size docker images to make CI faster.
• I used 9.0.1-11-slim tag because:
  • Java 9 is the latest released version of Java. There are no reason to use older release.
  • Other tags such as 9, 9.0, 9.0.1 are not appropriate for reproducibility. They can be implicitly replaced with minor released versions.

See also: #61

[mr-c]

Thank you!