Why My Dockerfile ARG Was Missing

I was doing some cleanup in a Dockerfile today when I noticed one of the packages was not on the correct version. It was on the latest version despite the fact there was an ARG pinning it…or so I thought. After some sleuthing, I found this helpful Stack Overflow post that explained and pointed me in the right direction.

Turns out…

“An ARG declared before a FROM…can’t be used in any instruction after a FROM.”

So the reason my package was installing the latest version was because the ARG didn’t exist! Conveniently, it also didn’t error, which is why it went unnoticed 🙃

The solution? Move the ARG down one line after the FROM.

# Bad
ARG VERSION=1.2.3
FROM ...

# Good
FROM ...
ARG VERSION=1.2.3