gplan.so: build from source using Raspbian Stretch Docker

Use balenalib/raspberry-pi-debian:stretch with legacy.raspbian.org repos.
Exact match: GCC 6.3, Python 3.5, GLIBC 2.24 — identical to the Pi.
First build ~25min (QEMU), subsequent builds ~1sec (cached image).

Replaces the broken Bullseye approach that had GLIBC/GLIBCXX mismatches.

.pi/Dockerfile.gplan

@@ -1,15 +1,24 @@
-# Pre-built armv7 environment for gplan.so
-# Build once:  docker build --platform linux/arm/v7 -t onefin-gplan -f .pi/Dockerfile.gplan .pi/
-# Then use:    .pi/build-gplan.sh
-FROM debian:bullseye
+# Raspbian Stretch armhf build environment for gplan.so
+# Matches the Pi exactly: GCC 6.3, Python 3.5, GLIBC 2.24
+#
+# Build image:  docker build -t onefin-gplan -f .pi/Dockerfile.gplan .pi/
+# Build gplan:  .pi/build-gplan.sh
+FROM balenalib/raspberry-pi-debian:stretch
 
-RUN apt-get update -qq && \
-    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends \
-      build-essential scons git ca-certificates python3-dev \
+# Fix repos to use archived Raspbian mirrors
+RUN echo "deb http://legacy.raspbian.org/raspbian/ stretch main contrib non-free rpi" \
+      > /etc/apt/sources.list && \
+    rm -f /etc/apt/sources.list.d/*.list
+
+RUN apt-get -o Acquire::Check-Valid-Until=false \
+      -o Acquire::AllowInsecureRepositories=true update && \
+    apt-get -o Acquire::Check-Valid-Until=false --allow-unauthenticated \
+      install -y --no-install-recommends \
+      build-essential python3-dev scons git ca-certificates \
       libssl-dev libexpat1-dev libbz2-dev liblz4-dev zlib1g-dev perl file && \
     rm -rf /var/lib/apt/lists/*
 
-# Clone and build cbang (static lib, ~5min under QEMU)
+# Clone and build cbang
 RUN mkdir -p /opt/cbang && cd /opt/cbang && git init -q && \
     git remote add origin https://github.com/CauldronDevelopmentLLC/cbang && \
     git fetch --depth 1 -q origin 18f1e963107ef26abe750c023355a5c40dd07853 && \
@@ -17,7 +26,7 @@ RUN mkdir -p /opt/cbang && cd /opt/cbang && git init -q && \
     scons -j2 disable_local="re2 libevent" && \
     rm -rf .git build/dep
 
-# Clone and patch camotics (source only, don't compile yet — that depends on workspace)
+# Clone, patch, and build camotics/gplan
 RUN mkdir -p /opt/camotics && cd /opt/camotics && git init -q && \
     git remote add origin https://github.com/CauldronDevelopmentLLC/camotics && \
     git fetch --depth 1 -q origin ec876c80d20fc19837133087cef0c447df5a939d && \
@@ -31,10 +40,9 @@ RUN mkdir -p /opt/camotics && cd /opt/camotics && git init -q && \
     done && \
     rm -rf .git
 
-# Pre-compile camotics objects (the slow part, ~20min under QEMU)
 ENV CBANG_HOME=/opt/cbang
-RUN cd /opt/camotics && scons -j2 gplan.so with_gui=0 with_tpl=0 && \
-    rm -f gplan.so  # remove the python3.9-linked one, we relink at build time
 
-ENV CBANG_HOME=/opt/cbang
+# Pre-compile everything including gplan.so
+RUN cd /opt/camotics && scons -j2 gplan.so with_gui=0 with_tpl=0
+
 WORKDIR /opt/camotics