MINIFI-541: Update Dockerfile to Follow Docker Best Practices

[SethFalco]

Thank you for submitting a contribution to Apache NiFi - MiNiFi.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with MINIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically master)?

• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi-minifi folder?
• N/A Have you written or updated unit tests to verify your changes?
• N/A If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• N/A If applicable, have you updated the LICENSE file, including the main LICENSE file under minifi-assembly?
• N/A If applicable, have you updated the NOTICE file, including the main NOTICE file found under minifi-assembly?

For documentation related changes:

• N/A Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[mattyb149]

I've taken the liberty of migrating this PR to the NiFi repo now that MiNiFi Java is integrated with the NiFi codebase. @SethFalco although the PR is on my fork/branch, you are still the author of the commit so when merged, the history will show your contribution