buildx multiarchitechture support fails for linux/arm/v7

[frakman1]

When attempting to build a docker image suitable for running on the Pi using the buildx multiarchitecture option for docker, I run into this error. Apparently, there is no debianslim-jre arm/v7 target.

$ docker buildx create --name multarchbuilder
$ docker buildx use multarchbuilder
$ docker buildx build -t tsunami-pi . --platform linux/arm/v7,linux/arm64/v8,linux/amd64

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 16.8s (9/9) FINISHED
 => [internal] booting buildkit                                                                                                                                                       12.6s
 => => pulling image moby/buildkit:buildx-stable-1                                                                                                                                    11.7s
 => => creating container buildx_buildkit_multarchbuilder0                                                                                                                             0.9s
 => [internal] load build definition from Dockerfile                                                                                                                                   0.1s
 => => transferring dockerfile: 1.35kB                                                                                                                                                 0.0s
 => [internal] load .dockerignore                                                                                                                                                      0.1s
 => => transferring context: 245B                                                                                                                                                      0.0s
 => CANCELED [linux/amd64 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim-jre                                                                                  3.9s
 => CANCELED [linux/amd64 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim                                                                                      3.9s
 => ERROR [linux/arm/v7 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim-jre                                                                                    3.8s
 => CANCELED [linux/arm/v7 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim                                                                                     3.9s
 => CANCELED [linux/arm64 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim-jre                                                                                  3.9s
 => CANCELED [linux/arm64 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim                                                                                      3.9s
------
 > [linux/arm/v7 internal] load metadata for docker.io/adoptopenjdk/openjdk13:debianslim-jre:
------
failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to create LLB definition: no match for platform in manifest sha256:a193ed5ea489b5ac33777010d7f12009ef9c0e0fc172181ab0365f07d4086137: not found