As you may guess, the ENV instruction is used to define environment variables that will be set in the running containers created from the image being built. The variables are defined using typical key-value pairs. A Dockerfile can have one or more ENV instructions. Here is the ENV instruction syntax:
# ENV instruction syntax
# This is the form to create a single environment variable per instruction
# Everything after the space following the <key> becomes the value
ENV <key> <value>
# This is the form to use when you want to create more than one variable per instruction
ENV <key>=<value> ...
Each ENV instruction will create one or more environment variables (unless the key name is repeated). Let's take a look at some ENV instructions in a Dockerfile:
# ENV instruction Dockerfile for Docker Quick Start
FROM alpine
LABEL maintainer...
