fix(docker): Use the latest alpine jdk8 image

[gounthar]

Update maven/jdk8/Dockerfile to use JDK8-based inbound agent image.
Fixes #46.

• Change the first stage base image from jenkins/inbound-agent:alpine to jenkins/inbound-agent:latest-alpine-jdk8
• Change the second stage base image from maven:3.6.3-jdk-8-slim to maven:3.8.6-jdk-8-slim.

Testing done

make && docker run -ti --rm jenkins/jnlp-agent-jdk8:latest

Submitter checklist

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