diff --git a/Dockerfile.arm64 b/Dockerfile.arm64
index beaf979..2462c50 100644
--- a/Dockerfile.arm64
+++ b/Dockerfile.arm64
@@ -1,13 +1,28 @@
-# TODO: support multi-arch building
-# TODO: https://docs.docker.com/reference/build-checks/from-platform-flag-const-disallowed/
+# Multi-arch Dockerfile for building Flutter Linux app images
+# - Uses BuildKit platform args to avoid hardcoding platforms in FROM
+# - Builds on the target architecture (via qemu when needed)
+# - Fixes: https://docs.docker.com/reference/build-checks/from-platform-flag-const-disallowed/
 
-# Multi-arch Dockerfile for building Flutter Linux app on ARM64 using emulation
-# Build with: docker buildx build --platform linux/arm64 -t pdf_signature_arm64 .
+# BuildKit-provided args available before FROM. Used to parameterize platforms.
+ARG TARGETPLATFORM
+ARG TARGETARCH
+ARG BUILDPLATFORM
+
+# Build with (single arch, locally loadable):
+#   docker buildx build --platform linux/amd64 -f Dockerfile.arm64 -t pdf_signature:amd64 --load .
+#   docker buildx build --platform linux/arm64 -f Dockerfile.arm64 -t pdf_signature:arm64 --load .
+# Build multi-arch (requires a registry):
+#   docker buildx build --platform linux/amd64,linux/arm64 -f Dockerfile.arm64 -t <your-registry>/pdf_signature:latest --push .
 
 # ----------------------------------------------------------------------------
 # Build stage (Ubuntu 18.04) — glibc 2.27 to maximize compatibility (<= 2.30)
 # ----------------------------------------------------------------------------
-FROM --platform=linux/arm64 ubuntu:18.04 AS build
+FROM ubuntu:18.04 AS build
+
+# Re-declare BuildKit args within the stage for RUN/CMD/ENV usage
+ARG TARGETPLATFORM
+ARG TARGETARCH
+ARG BUILDPLATFORM
 
 ENV DEBIAN_FRONTEND=noninteractive \
     TZ=UTC \
@@ -28,15 +43,21 @@ RUN apt-get update && apt-get install -y \
     liblzma-dev \
     && rm -rf /var/lib/apt/lists/*
 
-# Install a newer CMake (>= 3.13 required by Flutter linux build) for ARM64
+# Install a newer CMake (>= 3.13 required by Flutter linux build) per-arch
 ARG CMAKE_VERSION=3.27.9
-RUN curl -fsSL -o /tmp/cmake.tar.gz \
-      https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-linux-aarch64.tar.gz \
-    && tar -C /opt -xzf /tmp/cmake.tar.gz \
-    && ln -sf /opt/cmake-${CMAKE_VERSION}-linux-aarch64/bin/cmake /usr/local/bin/cmake \
-    && ln -sf /opt/cmake-${CMAKE_VERSION}-linux-aarch64/bin/ctest /usr/local/bin/ctest \
-    && ln -sf /opt/cmake-${CMAKE_VERSION}-linux-aarch64/bin/cpack /usr/local/bin/cpack \
-    && rm -f /tmp/cmake.tar.gz
+RUN set -eux; \
+    case "$TARGETARCH" in \
+        arm64) CMAKE_ARCH="aarch64" ;; \
+        amd64) CMAKE_ARCH="x86_64" ;; \
+        *) echo "Unsupported TARGETARCH for CMake: $TARGETARCH" >&2; exit 1 ;; \
+    esac; \
+    curl -fsSL -o /tmp/cmake.tar.gz \
+        https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-linux-${CMAKE_ARCH}.tar.gz; \
+    tar -C /opt -xzf /tmp/cmake.tar.gz; \
+    ln -sf /opt/cmake-${CMAKE_VERSION}-linux-${CMAKE_ARCH}/bin/cmake /usr/local/bin/cmake; \
+    ln -sf /opt/cmake-${CMAKE_VERSION}-linux-${CMAKE_ARCH}/bin/ctest /usr/local/bin/ctest; \
+    ln -sf /opt/cmake-${CMAKE_VERSION}-linux-${CMAKE_ARCH}/bin/cpack /usr/local/bin/cpack; \
+    rm -f /tmp/cmake.tar.gz
 
 # Install Flutter SDK (ARM64) from source repo (channel: stable)
 RUN git clone -b ${FLUTTER_CHANNEL} https://github.com/flutter/flutter.git /opt/flutter
@@ -67,13 +88,19 @@ RUN flutter pub get
 # Generate build_runner outputs if codegen files are referenced
 RUN bash -lc "if grep -R \"part '.*\\.g\\.dart'\|part '.*\\.freezed\\.dart'\" -n lib >/dev/null; then dart --disable-analytics >/dev/null 2>&1 || true; dart run build_runner build --delete-conflicting-outputs; fi"
 
-# Build the Linux app for ARM64 (explicitly specify, though container is ARM64)
-RUN flutter build linux --target-platform linux-arm64 --release
+# Build the Linux app for the target architecture (x64 on amd64; arm64 on arm64)
+RUN set -eux; \
+    case "$TARGETARCH" in \
+        amd64) FLUTTER_LINUX_TARGET="linux-x64" ;; \
+        arm64) FLUTTER_LINUX_TARGET="linux-arm64" ;; \
+        *) echo "Unsupported TARGETARCH for Flutter: $TARGETARCH" >&2; exit 1 ;; \
+    esac; \
+    flutter build linux --target-platform "${FLUTTER_LINUX_TARGET}" --release
 
 # ----------------------------------------------------------------------------
 # Runtime stage (Ubuntu 18.04) — matches glibc from builder (2.27)
 # ----------------------------------------------------------------------------
-FROM --platform=linux/arm64 ubuntu:18.04 AS runtime
+FROM ubuntu:18.04 AS runtime
 
 ENV DEBIAN_FRONTEND=noninteractive TZ=UTC
 
@@ -86,7 +113,8 @@ RUN apt-get update && apt-get install -y \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy the built app
-COPY --from=build /app/build/linux/arm64/release/bundle /app
+# Use wildcard to accommodate both x64 and arm64 bundle directories
+COPY --from=build /app/build/linux/*/release/bundle /app
 
 # Set working directory
 WORKDIR /app
diff --git a/docs/docker_compile.md b/docs/docker_compile.md
index 8e65738..4dc4700 100644
--- a/docs/docker_compile.md
+++ b/docs/docker_compile.md
@@ -7,20 +7,42 @@ docker run --privileged --rm tonistiigi/binfmt --install all
 docker buildx create --use --name multiarch
 ```
 
-build
+build (single-arch, locally loadable)
 
 ```bash
-docker buildx build --platform linux/arm64 -f Dockerfile.arm64 -t pdf_signature_arm64 --load .
+# amd64
+docker buildx build --platform linux/amd64 -f Dockerfile.arm64 -t pdf_signature:amd64 --load .
+
+# arm64
+docker buildx build --platform linux/arm64 -f Dockerfile.arm64 -t pdf_signature:arm64 --load .
 ```
 
-Extract the Built App
+build (multi-arch manifest, push to registry)
+
+```bash
+# requires a registry you can push to, e.g. ghcr.io/<org>/pdf_signature:latest
+docker buildx build \
+	--platform linux/amd64,linux/arm64 \
+	-f Dockerfile.arm64 \
+	-t <your-registry>/pdf_signature:latest \
+	--push .
+```
+
+Extract the Built App (from a single-arch image)
 
 ```bash
 mkdir output
-docker run --rm -v $(pwd)/output:/output pdf_signature_arm64 cp -r /app /output
+# select the architecture tag you built above
+# e.g. pdf_signature:arm64 or pdf_signature:amd64
+docker run --rm -v $(pwd)/output:/output pdf_signature:arm64 cp -r /app /output
 mkdir output/lib
-# docker run --rm -v $(pwd)/output:/output pdf_signature_arm64 ldd /app/pdf_signature
-docker run --rm -v $(pwd)/output:/output pdf_signature_arm64 sh -c "ldd /app/pdf_signature | grep '=>' | awk '{print \$3}' | grep -v libc | xargs -I {} cp -L {} /output/lib"
-# docker run --rm -v $(pwd)/output:/output pdf_signature_arm64 ls /app/lib
+# docker run --rm -v $(pwd)/output:/output pdf_signature:arm64 ldd /app/pdf_signature
+docker run --rm -v $(pwd)/output:/output pdf_signature:arm64 sh -c "ldd /app/pdf_signature | grep '=>' | awk '{print \$3}' | grep -v libc | xargs -I {} cp -L {} /output/lib"
+# docker run --rm -v $(pwd)/output:/output pdf_signature:arm64 ls /app/lib
 # tree output/
 ```
+
+Notes
+
+- The Dockerfile uses Ubuntu 18.04 to keep glibc at 2.27 for broad compatibility. If 18.04 mirrors are unavailable in your environment, consider switching to 20.04 (glibc 2.31) and testing on your target distros, or point apt to old-releases.
+- CMake binaries are fetched per-arch; Flutter target-platform is selected automatically (linux-x64 on amd64, linux-arm64 on arm64).