Docker Part II

Presented on 11th April 2021 by Jung

Dockerfile

Docker builds custom images by reading the instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands that assemble an image. Using docker build command, a user can automatically create a docker image that executes the custom command line insturctions successfully.

Here are some common instructions to use in a dockerfile.

ADD copies the files from a source on the host into the container’s own filesystem at the set destination.
CMD can be used for executing a specific command within the container. ENTRYPOINT sets a default application to be used every time a container is created with the image.
ENV sets environment variables.
EXPOSE associates a specific port to enable networking between the container and the outside world.
FROM defines the base image used to start the build process.
RUN is the central executing directive for Dockerfiles.
USER sets the UID (or username) which is to run the container.
VOLUME is used to enable access from the container to a directory on the host machine.
WORKDIR sets the path where the command, defined with CMD, is to be executed.
LABEL allows you to add a label to your docker image.

A Docker image consists of read-only layers each of which represents a Dockerfile insturction. The layer are stacked and each one is a delta of the changes from the previous layer. A user adds a new writable layer on top of the underlying layers. This is called the container layer.
RUN, COPY, ADD creates a layer. The other instructions will create intermediate layers and do not influence the size of your image.

To avoid losing data when a user destroy a docker container, Docker provides volumes and bind mounts mechanisms for persisting data in a Docker container.
Bind mounts mounts a file or directory on to your container from host machine, which can then reference via its absolute path. If a user wants a full control of the storage and plan on allowing other procsses besides Docker to access or modify the storage layer, then bind mounts is the right mechanism to use.
Bind mounts rely on the host machine’s filesystem having a specific directory structure available. THe path has to be specify explicitly to the file or folder to place the storage.
To user bind mounts on a container, two flags options are available to use, --mount and -v. --mount is more verbose and explicit so -v is a shorthand for --mount.

Container Volume is the preferred method for persisting data generated by and used by Docker containers.
According to the Docker documentation, volume is the easiest way to begin persisting data in a Docker container.
Volumes are completely handled by Docker which gives several advantages over bind mounts as stated below.
- Storage is not coupled to the lifecycle of the container but instead exists outside of it.
- Easier to back up or migrate.
- Can manage volumes by Docker CLI commands or the Docker API.
- Can be more safe to share among multiple containers.
- Volume drivers allow you to store on remote hosts or cloud providers.
- New volumes can have their content pre-populated by a container.
- Easy to attach to multiple running containers at the same time.
- The volumes don’t increase the size of the Docker container using them.

One of powerful advantage of docker containers is that it can be connected to non-docker workloads. A docker network allow docker containers to talk to its host, other containers on the host or any other machines on or outside the host’s network. docker network command provides subcommands such as ls, create, attach to configure networks and containers’ relationship.
Docker’s networking subsystem is pluggable using drivers. These are the drivers that exist by default.
bridge: The default network driver. It is usually used when an application run in standalone container that need to communicate.
host: For standalone containers, remove network isolation between the container and the Docker host, and use the host’s networking directly.
overlay: connects multiple Docker daemons together and enable swarm services to communicate with each other.
ipvlan: This drivker gives total control over both IPv4 and IPv6 addressing.
macvlan: allow to assign a MAC address to a container, making it appear as a physical device on your network.
none: This disables all networking.

A tool for defining and running multi-container Docker applications.
Use an YAMl file to configure your application’s services
Define the application stack in a file, and easily enable someone else to contribute to the project
Use a single command to spin everything up or tear it all down.
Usuaully defined in a file called docker-compose.yml and can spin up with command docker-compose up -d and docker-compose down to tear it down.