Docker

A runtime for containers.

A container is basically an isolated user space. Unlike traditional VMs, containers share the same OS.

Layers

A Docker image consists of read-only layers, each of which represents a Dockerfile instruction. Each layer is a delta of the changes from the previous layer. When a container is created from an image, a new writable layer is added on top of the underlying layers.

Storage

Types:

Volume: stored in a part of the host FS managed by Docker
Bind mount: stored somewhere on the host FS (non-Docker processes can modify it)

Both use the same -v flag, the difference is in the first argument: for a volume, it's the volume name; for a bind mount, it's a local directory path.

Dockerfile

Best practices:

Use WORKDIR instead of RUN cd ...
Use multi-stage builds
Exclude unwanted files from COPY with .dockerignore
Reduce the number of layers by minimizing the number of Dockerfile instructions
Use a separate RUN command to download dependencies (e.g. go mod download): unlike source code, they rarely change, so it's better to cache them in a separate layer

❓ Help
CMD or ENTRYPOINT? ENTRYPOINT should be used for the main command, CMD - for flags/arguments. Unlike ENTRYPOINT, CMD can be overwritten from the CLI.

Commands

docker build --tag=<name>:[tag] . - build an image from Dockerfile
docker run <image> [command] - create and run a container [with command]
- --name=<name> - assign a name to the container
- --detach - run the container in background
- --rm - remove the container when it is stopped
- -it - start a terminal session in the container
- -p=<host_port>:<container_port> - forward a port
- -v=<volume_name>:<container_path>:[mode] - mount a volume
docker logs [-f] <container> - print the container's log [and follow]
docker exec [-it] <container> <command> - run a command in the container
docker ps - list running containers
docker images - list downloaded images
docker rm <container> - remove the container
docker rmi <image> - remove the image
docker volume <create|ls|rm> - manage volumes
docker context <create|ls|rm|use|show> - manage contexts

Alternatives