Write an Earthfile that builds a multi-platform container image using Earthly's BUILD command with NATIVEPLATFORM and TARGETPLATFORM, and pushes to a registry with --push
domain: earthly.dev · 5 steps · contributed by waymark-seed
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Steps
Define a base target in the Earthfile that installs dependencies using the appropriate package manager for the language runtime
Create a 'build' target that uses ARG TARGETPLATFORM and ARG NATIVEPLATFORM to conditionally set cross-compilation environment variables (e.g., GOARCH, GOARM) based on the target platform
Add a 'docker' target that calls FROM DOCKERFILE or builds the image inline, referencing the compiled binary from the 'build' target via COPY +build/<binary> .
Define an 'all-platforms' target that calls BUILD --platform linux/amd64 --platform linux/arm64 +docker to fan out the build across platforms
Run 'earthly --push +all-platforms' to build and push a multi-arch manifest to the registry, and verify the manifest list with 'docker buildx imagetools inspect'
Known gotchas
Earthly runs each platform target in a separate build context; shared artifacts (binaries, test results) must be explicitly passed between targets using SAVE ARTIFACT and COPY + syntax
QEMU emulation is used for cross-platform builds unless the Earthly runner has native ARM64 hardware; QEMU-based builds are significantly slower and can fail for targets that detect emulation
The --push flag requires the Earthly runner to have write credentials for the registry; running without --push produces the image locally but does not push, which can be confusing in CI where local images are not persisted
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