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You're looking at a file literally called "Dockerfile" — no extension, no dot, just the name. It contains instructions for building a Docker container image: which base OS to start from, what software to install, which files to copy in, and what command to run when the container starts.
Dockerfiles use a simple declarative syntax. `FROM` picks the base image. `RUN` executes commands. `COPY` adds files. `CMD` sets the default startup command. Each instruction creates a cached layer, which means rebuilding after a small change is fast (only the changed layers rebuild). Getting the instruction order right — putting rarely-changing steps first — is the key to efficient Docker builds.
Any text editor opens Dockerfiles. VS Code with the Docker extension provides syntax highlighting, linting, and IntelliSense for Dockerfile instructions. To build an image: `docker build -t myapp .` in the directory containing the Dockerfile. Docker Desktop (free for personal use) provides the runtime. Never build a Dockerfile you haven't read — it can download and execute anything.