Docker "exec format error" - Fix Architecture/Shebang Mismatch in CI

The kernel could not execute the binary the container tried to run. Almost always the image (or an entrypoint binary) is built for a different CPU architecture than the host -- for example an arm64 image on an amd64 runner -- or a script has no valid shebang.

What this error means

The container fails immediately with exec /entrypoint: exec format error or standard_init_linux.go: exec user process caused: exec format error. It is deterministic for a given image+host pairing -- the architectures simply do not match.

docker

exec /usr/local/bin/app: exec format error
# or
standard_init_linux.go:228: exec user process caused: exec format error

Common causes

Image built for a different architecture

An image built for linux/arm64 is run on a linux/amd64 host (or vice versa). Without emulation the host kernel cannot execute the foreign binary.

Binary compiled for the wrong target

A statically compiled entrypoint targeted a different arch than the runner, so it cannot be exec'd even if the base image is correct.

Script missing a valid shebang

An entrypoint shell script with no #!/bin/sh line, or a CRLF-corrupted shebang, can also produce exec format error.

How to fix it

Build/pull for the host architecture

Match the image platform to the runner, or build multi-arch with buildx.

Terminal

# pull/run the right platform
docker run --platform linux/amd64 myorg/app:1.4.2
# build multi-arch
docker buildx build --platform linux/amd64,linux/arm64 -t myorg/app:1.4.2 --push .

Fix a binary or script target

Compile entrypoint binaries for the target architecture (set GOARCH/GOOS for Go).
Ensure entrypoint scripts start with a valid shebang and use LF line endings.
For cross-arch runs, register QEMU with docker/setup-qemu-action before running.

How to prevent it

Build multi-arch images with buildx when targets vary.
Pin --platform so an image always runs on a matching host.
Keep entrypoint scripts shebang-correct and free of CRLF endings.