fix(docker): Replace emsdk image with Ubuntu + emsdk from source

The Docker build used emscripten/emsdk:4.0.2-arm64 as base image but
env.sh couldn't find emsdk there, installing a second copy. The build
then applied wasm-opt/finalize stubs to the wrong emsdk (hardcoded
/emsdk/), so the real wasm-emscripten-finalize ran in Docker and got
OOM-killed.

- Use ubuntu:22.04 base with emsdk installed from source at /emsdk/
- Make stub paths dynamic via $EMSDK instead of hardcoded /emsdk/
- Skip local emsdk install in env.sh when $EMSDK is already active

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/Dockerfile

@@ -1,5 +1,7 @@
-# Use ARM64-native image for Apple Silicon (M1/M2/M3/M4)
-FROM emscripten/emsdk:4.0.2-arm64
+# Plain Ubuntu base — multi-arch (ARM64/x86_64 auto-detected)
+FROM ubuntu:22.04
+
+ENV DEBIAN_FRONTEND=noninteractive
 
 # Install build tools required for KiCad WASM build
 RUN apt-get update && apt-get install -y \
@@ -19,8 +21,17 @@ RUN apt-get update && apt-get install -y \
     unixodbc-dev \
     ccache \
     rsync \
+    xz-utils \
     && rm -rf /var/lib/apt/lists/*
 
+# Install emsdk from source (same approach as scripts/setup-emsdk.sh)
+# EMSCRIPTEN_VERSION must match scripts/common/versions.sh
+ARG EMSCRIPTEN_VERSION=4.0.2
+RUN git clone https://github.com/emscripten-core/emsdk.git /emsdk \
+    && cd /emsdk \
+    && ./emsdk install ${EMSCRIPTEN_VERSION} \
+    && ./emsdk activate ${EMSCRIPTEN_VERSION}
+
 # Configure ccache for Emscripten
 # CCACHE_DIR in named volume persists across containers
 ENV CCACHE_DIR=/workspace/build-wasm/.ccache