Why does one LABEL evaluate an expression in my Dockerfile and another does not?

[hughbris]

I'm seeing some inconsistent behaviour trying to populate my image labels using shell expressions. I hope it is obvious what I am trying to achieve from my descriptions:

LABEL org.opencontainers.image.version="$(git describe --tags --abbrev=0)"
LABEL org.opencontainers.image.revision="$(git rev-parse HEAD)"

results in these labels in the image when I dump them:

"org.opencontainers.image.version" : "0.2.6"
"org.opencontainers.image.revision" : "$(git rev-parse HEAD)"

That is, one expression is evaluated, the other not!

I also tried this with LABEL org.opencontainers.image.revision="$(git describe --tags --abbrev=0)" and even …="$(pwd)" and the shell command is also not evaluated, so I suspect it is not the expression $(git rev-parse HEAD) that is the issue here.

Build platform information:

• Linux
• zsh (the git commands have been tested to work)
• buildx version 0.19.2 1fc5647dc281ca3c2ad5b451aeff2dce84f1dc49

Happy to provide any other information required to diagnose this, or transform this into a bug issue if suitable.

(I am aware that I could pass these expressions or values in a build script with --label, but that's not what I want to do here.)

[hughbris]

Quite embarrassed now, realising 0.2.6 was in fact hardcoded as the default in my build shell script: VER=${3:-0.2.6} … --label org.opencontainers.image.version=$VER before I decided to work on making this dynamic.

I guess shell expression expansion is not supported at all in Dockerfile LABEL :/

Sorry if I wasted anyone's time trying to reproduce.