Docker CLI reference covering containers, images, Compose, volumes, networks, troubleshooting, and Dockerfile optimization. Placed in optional-skills/devops/ since it's a documentation-only skill with no external dependencies beyond Docker CLI. Based on PR #3032 by @sprmn24. Moved from skills/ to optional-skills/ and trimmed the description to be concise. Co-authored-by: sprmn24 <sprmn24@users.noreply.github.com>