Small script to pair with the Dockerfile ADD directive to make an idempotent action regardless of if the <src> of the ADD is a local tar archive or a tar archive from a remote URL.
A Dockerfile's ADD directive is a bit finicky. It allows files to be added to the image either from a local directory or a remote URL. This has value in that if the <src> of the ADD is an ARG, then the functionality of the ADD can be changed with build-time arguments.
For example, this allows developers to lazily get the latest version of the dependency from a tar archive at some remote URL while CI consistently supplies the same tar archive to the Dockerfile as a --build-arg.
However, if that <src> is expected to be either a local tar archive or a remote tar archive, the ADD directive is not idempotent; ADD unpacks local tar archives but not remote ones. As a result, subsequent Dockerfile directives would have to account for the difference to make the build successful.
This is where the ADDendum comes in:
FROM scratch
COPY --from=ghcr.io/frantjc/dockerfile-addendum /addendum /usr/local/bin
ARG tgz=zip_3.0_x86_64.tgz
ADD ${tgz} /tmp
RUN addendum -ruo /usr/local/bin /tmp/$(basename ${tgz})Now this Dockerfile can be built with --build-arg tgz=<src> where <src> is either a tar archive at a remote URL or a local tar archive.