Assisted-by: Claude Opus 4.6 via pi

diff --git a/assets/Dockerfile b/assets/Dockerfile
index 8ff223a..64fa6cd 100644
--- a/assets/Dockerfile
+++ b/assets/Dockerfile
@@ -1,5 +1,7 @@
 FROM node:24-trixie-slim
 
+ARG TARGETARCH
+
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
     curl \
@@ -9,6 +11,15 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/* \
     && ln -sf /usr/bin/fdfind /usr/bin/fd
 
+ARG JJ_VERSION=0.39.0
+RUN case "${TARGETARCH}" in \
+      amd64) JJ_ARCH="x86_64" ;; \
+      arm64) JJ_ARCH="aarch64" ;; \
+      *) echo "unsupported architecture: ${TARGETARCH}" && exit 1 ;; \
+    esac && \
+    curl -fsSL "https://github.com/jj-vcs/jj/releases/download/v${JJ_VERSION}/jj-v${JJ_VERSION}-${JJ_ARCH}-unknown-linux-musl.tar.gz" \
+      | tar xz -C /usr/local/bin jj
+
 RUN npm install -g @mariozechner/pi-coding-agent
 
 WORKDIR /workspace