Docker Multistage Build ARGs are no longer a mystery. Read to find out why.

At Bobcares, we offer solutions for every query, big and small, as a part of our Docker Hosting Support Services.

Let’s look at how our Docker Support Team is ready to help customers understand Docker Multistage Build ARGs.

What are Docker Multistage Build ARGs?

A Docker multistage build is responsible for allowing us to use several images to build a final product. Although there is a single Dockerfile, it defines several images inside, enabling us to build the final image.

ARG or built-time variables are available from the instant they are announced in the Dockerfile along with an ARG instruction. Additionally, they are available till the instant the image is built. However, running containers do not have access to ARG variable values.

FROM debian as base
# ARD declaration
ARG THIS_IS_ARG=default_arg_value
# Echo its value at build time
RUN echo "I am in the base image during build time and THIS_IS_ARG value is ${THIS_IS_ARG:-}"
# It is undeclared at runtime

docker build -f Dockerfile --progress=plain --no-cache -t tester .
docker build -f Dockerfile --progress=plain --no-cache -t tester --build-arg THIS_IS_ARG=overridden .
docker run --rm tester

As seen above, ARG syntax is :

ARG [=<default value>]

It defines a variable that we can pass during build-time to the builder via the docker build command with the ARG <name>[=<default value>] flag.

Moreover, a dockerfile may contain more than one ARG instruction as well. For instance:

FROM sembox
ARG user1
ARG buildno
# ...

In case an ARG instruction consists of a default value and no value s at build-time, the builder uses the default value.

It is critical to note that build-time variables are visible to all image users via the docker history command. Hence, our Docker Support experts recommend avoiding build-time variables for passing secrets like user login details, github keys, and so on.

We inherit all ENV as well as ARG defined in all previous stages in multi-stage builds. In case we override an ARG during build-tie, all stages receive the new value. However, ENV cannot be modified at build time.

In case we want to parameterize the FROM image, Docker permits us to declare ARG before the first FROM at the top of the Dockerfile. For instance:

ARG FROM_IMAGE=debian
FROM $FROM_IMAGE

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Conclusion

In a nutshell, our skilled Docker Support Engineers at Bobcares introduced us through Docker Multistage Build ARGs.