Investigate if it's feasible to use local Docker or tar base image when building multi-platform images

[chanseokoh]

[chanseokoh]

About local Docker daemon images:

Note it is possible to pull an image to a Docker daemon even if the image platform doesn't match the platform running the daemon. (Of course, you can't run an image that doesn't match the machine platform.)

However, I doubted a local Docker daemon could host something like a manifest list. Tried buildx by following the doc to build multi-arch images but without pushing to a registry (i.e., without --push). As I expected, it shows a warning "no output specified".

$ docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7 .
WARN[0000] No output specified for docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load 
[+] Building 24.0s (9/9) FINISHED                                                                                                                                                                                 
 => [internal] booting buildkit                                                                                                                                                                             16.7s
 => => pulling image moby/buildkit:buildx-stable-1                                                                                                                                                          15.5s
 => => creating container buildx_buildkit_mybuilder0                                                                                                                                                         1.2s
...

It's not feasible to use a base image from a Docker daemon when configuring multiple <platform>s at the moment. Potentially we could change our <from><image> config so that the user can configure multiple local Docker images with different architectures. However, I am not sure if this is worth considering. For now, I think we can just fail when given multiple platforms.

[chanseokoh]

About local tar images:

Haven't looked into it, but I speculate that a tar image (either OCI or Docker) can easily contain multiple images. Worth taking a look.

[chanseokoh]

But for now, I think these are a low priority and we can just fail when attempting to use a local base image with multiple platforms configured. Should be done before we can enable multi-platform image building.

[tom-haines]

added some local tar buildx image info on issue #2751 if relevant

[chanseokoh]

@tom-haines thanks for the info!

[SgtSilvio]

Using an OCI image tarball or even directory as a base image would make a lot of sense. Related: #2173

[hypnoce]

It would also be of great help to output the built image into an oci directory structure instead of an oci tar, amending any existing index.