Update Dockerfile and fix layers caching

[LukasForst]

I updated the dockerfile - replaced ADD with RUN and thus allowed caching of the Dockerize.

RUN hashes just the URL, on the other hand ADD computes hash from the downloaded file so the file must be downloaded every time.

Step 1/12 : FROM openjdk:8-jre-alpine

 ---> f7a292bbb70c
Step 2/12 : VOLUME /tmp

 ---> Using cache
 ---> 674131380226
Step 3/12 : ARG DOCKERIZE_VERSION

 ---> Using cache
 ---> 4e71f5a79327
Step 4/12 : RUN wget -O dockerize.tar.gz https://github.com/jwilder/dockerize/releases/download/${DOCKERIZE_VERSION}/dockerize-alpine-linux-amd64-${DOCKERIZE_VERSION}.tar.gz

 ---> Using cache
 ---> f04603ca89a8
Step 5/12 : RUN tar xzf dockerize.tar.gz

 ---> Using cache
 ---> b3b45f125cc1
Step 6/12 : RUN chmod +x dockerize

 ---> Using cache
 ---> 06b6c870566e
Step 7/12 : ARG ARTIFACT_NAME

 ---> Using cache
 ---> 834646a63ccb

This fixes #136

[arey]

Thanks a lot @LukasForst