Can I build with multiple compression types?

[gapuchi]

I wanted to build two images, one compressed with gzip and one with zstd, but I wanted to ensure that the images are exactly the same.

The concern with running two build commands right after another is that the base image may change between the two builds.

So I was wondering if there was a way for me to build two images at once, or take one image and uncompress and recompress it, or any other way to achieve this.

[tonistiigi]

There is no way to specify multiple compression types for same build.

When builds are invoked in parallel(second invoked before first completes) they are guaranteed to get the same base image.

Other option is to use buildx bake with a context pointing to the previous build that just returns the context itself. Then you can have many bake targets with their own exporter options building the same thing. Last section https://www.docker.com/blog/dockerfiles-now-support-multiple-build-contexts/

[jedevc]

Eventually, I think we should be able to support multiple exporters, see here for some in-progress work.

With multiple exporters, you'd be able to generate two separate images with different compression types, but with the same layers.

[gapuchi]

I see, if I were to use bake, I'd need a second, minimal Dockerfile for the second build to use, where the base image is the context provided by the first target?

[tonistiigi]

Yes, or a different target from the same Dockerfile. Bake also supports inline dockerfiles.

[gapuchi]

Great, thanks! Just to confirm my understanding, if I were to have two targets using the same Dockerfile, running bake with those two targets in a single command would result in the builds occurring in parallel and would ensure we have the same layers?

It'd be good to avoid the second Dockerfile with this approach.