feat: add Dockerfile for scanning Docker repos

[lfdebrux]

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

What does this PR do?

Add a Dockerfile for generating a Docker image with the Snyk CLI and the Docker CLI.

This means that a Docker user can scan Docker images using a Docker image!

Where should the reviewer start?

Build with

docker build -t snyk/snyk-cli:docker -f docker/Dockerfile.docker docker

How should this be manually tested?

Run using a command such as

docker run --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=docker/Dockerfile.docker --json

Any background context you want to provide?

I want to run Snyk tests on Docker images generated on CI, and it's easier to have Snyk in a container than install it on the CI server.

Additional questions

Can this also be added to the build system with the other Docker images?

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}