Recently I had to migrate some services from amd64 instances to arm64 instances, specifically AWS Graviton. These services needed to install their dependencies differently when building for amd64 and arm64.

My goal was to use a single Dockerfile to build both amd64 and arm64 images, with minimal to no changes in the build steps; it should be as automatic as possible.

Solution 1: Run commands dynamically

The most obvious solution would be to run commands based on the architecture of the build machine.

FROM alpine:3.21 AS base

RUN ARCH=$(uname -m) \
  && if [ "$ARCH" = "x86_64" ]; then \
  echo "Do something for amd64"; \
  elif [ "$ARCH" = "aarch64" ]; then \
  echo "Do something else for arm64"; \
  fi

Or you can use the TARGETARCH argument if you are using BuildKit. This argument will be injected automatically during the build and will be carried to subsequent stages.

FROM alpine:3.21 AS base
ARG TARGETARCH

RUN if [ "$TARGETARCH" = "amd64" ]; then \
  echo "Do something for amd64"; \
  elif [ "$TARGETARCH" = "arm64" ]; then \
  echo "Do something else for arm64"; \
  fi

Solution 2: Use a middleman image

Some operations need to happen in the Dockerfile itself rather than in the build container. Take setting environment variables as an example. You cannot do RUN export SOME_VAR="some value" and expect it to be carried to the next build stage; it is not.

For this, we can create a ‘middleman’ image for adding something on top of the next stage and utilise BuildKit’s TARGETARCH to select the middleman image as the base image dynamically based on the architecture.

The following example sets the path for LD_PRELOAD differently for each architecture. Notice how the base image is chosen dynamically through FROM base-${TARGETARCH}.

FROM alpine:3.21 AS base
ARG TARGETARCH

# Do something

FROM base AS base-amd64
ENV LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.2

FROM base AS base-arm64
ENV LD_PRELOAD=/usr/lib/aarch64-linux-gnu/libjemalloc.so.2

FROM base-${TARGETARCH} AS release

# Do something

Please feel free to share your solutions!