Pratical example on how to docckerize various web applications.
What is Docker ?
Docker is a platform for consistently building, running, and shipping applications
| Container | Virtual Machine |
|---|---|
| A container is an isolated environment for running an application. It’s essentially an operating-system process with its own file system | A virtual machine is an abstraction of hardware resources. Using hypervisors we can create and manage virtual machines. The most popular hypervisors are VirtualBox, VMware and Hyper-v (Windows-only). |
| Containers are very lightweight and start quickly because they share the kernel of the host (which is alreadystarted). | Virtual machines are very resource intensive and slow to start |
Docker Architecture
Docker uses client/server architecture. It has a client component that talks to the server using a RESTful API. The server is also called the Docker engine (or daemon) runs in the background and is responsible for doing the actual work
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Using Docker, we can bundle an application into an image. Once we have an image, we can run it on any machine that runs Docker.
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An image is a bundle of everything needed to run an application. That includes a cutdown OS, a runtime environment (eg Node, Python, etc), application files, thirdparty libraries, environment variables, etc.
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To bundle an application into an image, we need to create a Dockerfile. A Dockerfile contains all the instructions needed to package up an application into an image
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We can share our images by publishing them on Docker registries. The most popular Docker registry is Docker Hub
Dockerfile instructions
- FROM # to specify the base image
- WORKDIR # to set the working - directory
- COPY # to copy files/directories
- ADD # to copy files/directories
- RUN # to run commands
- ENV # to set environment variables
- EXPOSE # to document the port the container is listening on
- USER # to set the user running the app
- CMD # to set the default command/program
- ENTRYPOINT # to set the default command/program
Image commands
- docker build -t .
- docker images
- docker image ls
- docker run -it
sh
Starting and stopping containers
- docker stop
- docker start <containerID
Removing containers
- docker container rm
- docker rm
- docker rm -f # to force the removal
- docker container prune # to remove stopped containers
Volumes
- docker volume ls
- docker volume create app-data
- docker volume inspect app-data
- docker run -v app-data:/app/data
Copying files between the host and containers
- docker cp :/app/log.txt .
- docker cp secret.txt :/app
Sharing source code with containers
- docker run -v $(pwd):/app