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Docker

About

Docker is a tool used to automate the deployment of applications in lightweight containers so that applications can work efficiently in different environments.

Start container

We can start a container with the following command:

docker run [<options>] <image> [<arguments_for_image>]

By default, if you specify only image name, Docker pulls image from Docker Hub — largest library and community for container images.

Start “nginx” container

Let’s start official nginx image:

docker run --rm -it -p 8080:80 nginx

... [notice] ...: nginx/...
... [notice] ...: start worker processes
...

Used docker run options:

  • --rm: Automatically remove the container when it exits.
  • -i / --interactive: Keep STDIN open even if not attached.
  • -t / --tty: Allocate a pseudo-TTY.
  • -p / --publish: Publish a port from container to the host (<host_port>:<container_port>). Use multiple times for multiple ports.

Let’s list containers:

docker ps

CONTAINER ID IMAGE COMMAND CREATED       STATUS       PORTS                NAMES
11aa22bb33cc nginx ...     5 seconds ago Up 5 seconds 0.0.0.0:8080->80/tcp random_name_nginx

Let’s connect to the started nginx via curl:

curl localhost:8080

<!DOCTYPE html>
...
<title>Welcome to nginx!</title>
...

Start “PostgreSQL” container

Let’s start official PostgreSQL image:

docker run --rm -it -p 15432:5432 -e POSTGRES_PASSWORD=password \
           -v $HOME/postgres_volume:/var/lib/postgresql/data postgres

...
PostgreSQL init process complete; ready for start up.
...
... database system is ready to accept connections

New used docker run options:

  • -e / --env: Environment variables. Use multiple times for multiple variables.
  • -v / --volume: Bind mount a volume (<host_path>:<container_path>, absolute paths must be used). Use multiple times for multiple volumes.

Let’s list containers:

docker ps

CONTAINER ID IMAGE    COMMAND CREATED       STATUS       PORTS                   NAMES
aa11bb22cc33 postgres ...     5 seconds ago Up 5 seconds 0.0.0.0:15432->5432/tcp random_name_postgres

Let’s connect to the started PostgreSQL via psql command:

PGPASSWORD=password psql --host localhost --port 15432 --user postgres --command "SELECT version()"

    version
---------------
 PostgreSQL ...

Connect to your host from the container

You can use the special DNS name host.docker.internal to connect to your host from the container (works on Mac and Windows). On Linux, add --add-host=host.docker.internal:host-gateway to your Docker command to enable this feature.

Let’s connect to our started PostgreSQL via psql command from another container:

docker run --rm -it -e PGPASSWORD=password postgres \
       psql --host host.docker.internal --port 15432 --user postgres --command "SELECT version()"

    version
---------------
 PostgreSQL ...