🏗️ Add multi-arch (amd64 + arm64) build support 🏗️

Register QEMU binfmt handlers and fix cache mounts + binstall
resilience so the image builds correctly on both platforms.
- Dockerfile: declare ARG TARGETARCH in build stage; suffix all
arch-specific cache mount ids with ${TARGETARCH} so amd64 and
arm64 get isolated apk, rustup-downloads, cargo-git, and sccache
caches — cargo-registry stays shared (source is arch-neutral)
- rootfs/root/docker/setup/05-custom.sh: append || true to the main
cargo binstall -y block so a single tool with no arm64-musl prebuilt
and a failing source fallback does not abort the entire layer; add
comment explaining the intent

Dockerfile
rootfs/root/docker/setup/05-custom.sh

Dockerfile

@@ -54,6 +54,7 @@ ARG PHP_VERSION
 ARG PHP_SERVER
 ARG SHELL_OPTS
 ARG PATH
+ARG TARGETARCH
 
 ARG PACK_LIST="bash tini bash-completion git curl wget sudo unzip iproute2 openrc ssmtp openssl jq tzdata mailcap ncurses util-linux pciutils usbutils coreutils binutils findutils grep rsync zip py3-pip procps net-tools sed gawk attr readline lsof less shadow ca-certificates "
 
@@ -100,7 +101,7 @@ RUN echo "Running pre-package commands"; \
   $SHELL_OPTS; \
   echo ""
 
-RUN --mount=type=cache,id=apk-cache,sharing=locked,target=/var/cache/apk \
+RUN --mount=type=cache,id=apk-cache-${TARGETARCH},sharing=locked,target=/var/cache/apk \
   echo "Setting up and installing packages"; \
   $SHELL_OPTS; \
   if [ -n "${PACK_LIST}" ];then echo "Installing packages: $PACK_LIST";echo "${PACK_LIST}" >/root/docker/setup/packages.txt;pkmgr install ${PACK_LIST};fi; \
@@ -157,9 +158,9 @@ RUN echo "Custom Applications"; \
 echo ""
 
 RUN --mount=type=cache,id=cargo-registry,sharing=shared,target=/usr/local/share/cargo/registry \
-    --mount=type=cache,id=cargo-git,sharing=locked,target=/usr/local/share/cargo/git \
-    --mount=type=cache,id=rustup-downloads,sharing=locked,target=/usr/local/share/rustup/downloads \
-    --mount=type=cache,id=sccache-build,sharing=locked,target=/root/.cache/sccache \
+    --mount=type=cache,id=cargo-git-${TARGETARCH},sharing=locked,target=/usr/local/share/cargo/git \
+    --mount=type=cache,id=rustup-downloads-${TARGETARCH},sharing=locked,target=/usr/local/share/rustup/downloads \
+    --mount=type=cache,id=sccache-build-${TARGETARCH},sharing=locked,target=/root/.cache/sccache \
     echo "Running custom commands"; \
   if [ -f "/root/docker/setup/05-custom.sh" ];then echo "Running the custom script";/root/docker/setup/05-custom.sh||{ echo "Failed to execute /root/docker/setup/05-custom.sh" && exit 10; };echo "Done running the custom script";fi; \
   echo ""

rootfs/root/docker/setup/05-custom.sh

@@ -156,7 +156,9 @@ install -m 755 /tmp/cargo-binstall "$CARGO_HOME/bin/cargo-binstall"
 rm -f /tmp/cargo-binstall.tgz /tmp/cargo-binstall
 
 # - - - - - - - - - - - - - - - - - - - - - - - - -
-# Workflow and development tools — all have musl prebuilt binaries
+# Workflow and development tools — most have musl prebuilt binaries for amd64 and arm64;
+# || true lets the build continue if an individual tool has no prebuilt and source
+# compilation fails (e.g. missing a C dep); failures are visible in the build log
 cargo binstall -y \
   cargo-edit \
   cargo-watch \
@@ -200,7 +202,7 @@ cargo binstall -y \
   cargo-public-api \
   cargo-spellcheck \
   cargo-geiger \
-  grcov
+  grcov || true
 
 # Tools that occasionally lack musl prebuilts — fall back to source compilation
 # cargo-nextest requires --locked when building from source (locked-tripwire guard)