Question 1

What is the difference between a Docker image and a container?

Accepted Answer

An image is a read-only template that contains the app and its dependencies. A container is a running instance of that image. One image can start many containers. Each container has its own writable layer, so changes inside it do not change the image. Removing a container does not remove the image it came from.

Question 2

What does EXPOSE do in a Dockerfile?

Accepted Answer

EXPOSE documents which port the app listens on. It does not publish the port to your computer. To make the port reachable from the host, you publish it at run time with the -p flag, for example -p 8000:8000. EXPOSE is useful documentation, and it lets tools know which port to expect.