How to link containers?

To link Docker containers, you can use the legacy --link flag or modern Docker networking features. Here’s a detailed explanation of both methods:

Using the Legacy --link Flag

The --link flag is a legacy feature that allows you to link multiple containers together, enabling them to share information securely. Here’s how you can link containers using this method:

1. Create the Source Container:
   First, create a container that will act as the source. For example, a PostgreSQL database container:

   $ docker run -d --name db training/postgres

2. Remove the Existing Recipient Container:
   If you have an existing container that you want to link, remove it first:

   $ docker container rm -f web

3. Create and Link the Recipient Container:
   Create a new container and link it to the source container using the --link flag:

   $ docker run -d -P --name web --link db:db training/webapp python app.py

   In this command, --link db:db links the web container to the db container, allowing web to access information about db.

4. Inspect the Link:
   You can inspect the link to verify the connection:

   $ docker inspect -f "{{ .HostConfig.Links }}" web

   This should show the link between the web and db containers.

5. Accessing Linked Container Information:
   The recipient container can access the source container’s information via environment variables and the /etc/hosts file. For example, the web container can access the db container’s IP address and other details without exposing ports externally[1][4].

Using Modern Docker Networking

Modern Docker networking provides more flexible and powerful ways to connect containers. Here’s how you can achieve container communi...