Pin the base docker image's JRE version (#1422)
Pin the base docker image's JRE version (#1422)

Problem

By depending on the "java:openjdk-8-jre" tag, docker just uses whatever
minor version of the JRE is available the first time that image is
pulled. This can lead to unpredictable minor versions across releases.
For instance:

```
$ docker run --rm --entrypoint=sh buoyantio/linkerd:0.9.1 -c 'echo $JAVA_DEBIAN_VERSION'
8u111-b14-2~bpo8+1
$ docker run --rm --entrypoint=sh buoyantio/linkerd:1.0.0 -c 'echo $JAVA_DEBIAN_VERSION'
8u102-b14.1-1~bpo8+1
$ docker run --rm --entrypoint=sh buoyantio/linkerd:1.0.2 -c 'echo $JAVA_DEBIAN_VERSION'
8u111-b14-2~bpo8+1
```

Solution

Pin our base image to a known recent minor version: "openjdk:8u131-jre".
As part of this change I'm also switching us from the "java" image to
the "openjdk" image, since "java" is deprecated.
denverwilliams/linkerd,linkerd/linkerd,denverwilliams/linkerd,BuoyantIO/linkerd,BuoyantIO/linkerd,denverwilliams/linkerd,denverwilliams/linkerd,linkerd/linkerd,linkerd/linkerd