Learn Docker/Writing a Dockerfile/EXPOSE instruction - Missing data and mismatched port information

[juanchax]

• The first line of the EXPOSE instruction explanation is missing part of the information.
• The line below EXPOSE 80/udp states that we want to match the port exposed in our container with the one we have in src/index.js, but on the js file is 3000 and on the Dockerfile example provided it's 8000. Shouldn't it be the same?

EXPOSE instruction

EXPOSE specifies which ports the **<missing data>** when running the container. Each port is separated by a space ( ). By default the container listens using TCP, but UDP can be used by adding /udp after the port.

For example:

EXPOSE 80/udp

In our case, we want the port exposed by our container to match the port our application is running on (remember: const port = process.env.PORT || 3000 in src/index.js), therefore, we are publishing the value of our PORT environment variable.

ARG PORT=8000
ENV PORT=$PORT
...
EXPOSE $PORT

EXPOSE does not actually publish the port. When running the container with docker run, the -p flag needs to be used to access the port from your host machine. More on this in the next section. Learn more about EXPOSE