Additional tags for result image

[antoinedeschenes]

Image rust-buster-1-stable-x86_64 could also be tagged with rust-buster-1-stable-amd64 and
image rust-buster-1-stable-aarch64 could also be tagged with rust-buster-1-stable-arm64.

This would allow child image Dockerfiles to use the BuildKit TARGETARCH argument:
FROM --platform=$BUILDPLATFORM docker.io/logdna/build-images:rust-buster-1-stable-$TARGETARCH as build

Example use case is when using docker buildx with multiple platforms, where one can't use platform-dependant build-args:
docker buildx build --build-arg MYARG=value --platform linux/arm64,linux/amd64 -t myimage ..