Allow skipping Docker build from main build process

[porunov]

Currently on a system with disabled docker if you try to build a project with the following command:

mvn clean install -Pjanusgraph-release -DskipTests=true --batch-mode --also-make -Dgpg.skip=true

You will get an exception:

ERROR: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

In some situations users don't need to build the docker image during JanusGraph build process. Thus, -Pskip-docker argument is added to allow skipping Docker image build.
I.e. the following command is passing normally and JanusGraph artifacts are built as expected:

mvn clean install -Pjanusgraph-release -Pskip-docker -DskipTests=true --batch-mode --also-make -Dgpg.skip=true

---

Thank you for contributing to JanusGraph!

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 an issue associated with this PR? Is it referenced in the commit message?
• Does your PR body contain #xyz where xyz is the issue number you are trying to resolve?
• 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 written and/or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE.txt file, including the main LICENSE.txt file in the root of this repository?
• If applicable, have you updated the NOTICE.txt file, including the main NOTICE.txt file found in the root of this repository?

For documentation related changes:

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