dockerfiles.md

Dockerfile

Change base image by argument

Sometime you need to change the base image at build time. For example to use an non-enterprise image for testing purposes. This can easily be done by using arguments.

Understand how ARG and FROM interact

Excerpt from docs.docker.com

FROM instructions support variables that are declared by any ARG instructions that occur before the first FROM.

ARG  CODE_VERSION=latest
FROM base:${CODE_VERSION}
CMD  /code/run-app

FROM extras:${CODE_VERSION}
CMD  /code/run-extras

An ARG declared before a FROM is outside of a build stage, so it can’t be used in any instruction after a FROM. To use the default value of an ARG declared before the first FROM use an ARG instruction without a value inside of a build stage:

ARG VERSION=latest
FROM busybox:$VERSION
ARG VERSION
RUN echo $VERSION > image_version