Add Dockerfile.docker and update README.md

[odinn1984]

• Ready for review
• Follows CONTRIBUTING rules
• Reviewed by Snyk internal team

What does this PR do?

Contributed by: #266

Where should the reviewer start?

How should this be manually tested?

cd snyk/docker
docker build -f Dockerfile.docker -t snyk/snyk-cli:docker .
docker run -it --rm -e SNYK_TOKEN=$SNYK_TOKEN -v $PWD:/project -v /var/run/docker.sock:/var/run/docker.sock snyk/snyk-cli:docker test --docker snyk/snyk-cli:docker --file=/project/Dockerfile.docker

Any background context you want to provide?

What are the relevant tickets?

Screenshots

Additional questions

[snyksec]

🎉 This PR is included in version 1.138.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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