Install Seafile CLI from AppImage instead of the APT repository

Seafile discontinued linux-clients.seafile.com: it resolves but accepts
no connections on 80 or 443, and the official docs no longer mention APT
at all. Every `docker build` has failed since. Upstream now ships the CLI
as an AppImage, so install 9.0.19 from the download site.

The AppImage is unpacked rather than executed. Running one needs FUSE and
so a privileged container, and its self-extraction also fails when the
build host cannot execute the payload's architecture. Unpacking the
appended squashfs directly avoids both.

Also add the bundled seafile/pysearpc bindings to the venv path. The APT
package installed them system-wide, where `venv --system-site-packages`
picked them up; dsc imports seafile directly for RPC, not only through
seaf-cli, so it broke when that implicit dependency went away.

Drops apt-key, pins the version, and verifies a SHA256 before unpacking.
Build-time smoke checks assert the seaf-cli subcommands and the app's own
imports.

Note: the AppImage is x86_64-only, and upstream publishes no digest or
signature, so the pinned checksum is trust-on-first-use.

Dockerfile

@@ -1,21 +1,54 @@
 FROM debian:bookworm-slim
 
-# Install seafile client
+# Seafile discontinued its APT repository; linux-clients.seafile.com no longer
+# resolves to a reachable host. Upstream now ships the CLI as an AppImage.
+ARG SEAFILE_CLI_VERSION=9.0.19
+ARG SEAFILE_CLI_SHA256=4af848362d8493be218b903e17e3fbbd282e9ea94af000211ce05c3f44e48715
+ARG SEAFILE_CLI_URL=https://sos-ch-dk-2.exo.io/seafile-downloads/Seafile-cli-x86_64-${SEAFILE_CLI_VERSION}.AppImage
+
 RUN apt-get update && \
-    apt-get install gnupg curl python3.11-venv -y && \
-    rm -rf /var/lib/apt/lists/*
-RUN curl https://linux-clients.seafile.com/seafile.asc | apt-key add - && \
-    echo 'deb [arch=amd64] https://linux-clients.seafile.com/seafile-deb/bookworm/ stable main' \
-    > /etc/apt/sources.list.d/seafile.list && \
-    apt-get update -y && \
-    apt-get install -y seafile-cli && \
+    apt-get install -y --no-install-recommends \
+        ca-certificates curl binutils squashfs-tools python3.11-venv && \
     rm -rf /var/lib/apt/lists/*
 
+# An AppImage is an ELF runtime with a squashfs image appended. Unpack the
+# squashfs directly rather than executing the AppImage: running it needs FUSE,
+# and so a privileged container. Verify the download before unpacking it.
+RUN curl -fsSL -o /tmp/seaf-cli.AppImage "$SEAFILE_CLI_URL" && \
+    echo "${SEAFILE_CLI_SHA256}  /tmp/seaf-cli.AppImage" | sha256sum -c - && \
+    offset=$(LC_ALL=C readelf -h /tmp/seaf-cli.AppImage | awk '\
+        /Start of section headers/ {s=$5} \
+        /Size of section headers/  {z=$5} \
+        /Number of section headers/{n=$5} \
+        END {print s + z * n}') && \
+    unsquashfs -q -d /opt/seafile-cli -o "$offset" /tmp/seaf-cli.AppImage && \
+    rm /tmp/seaf-cli.AppImage
+
+# The bundled AppRun expects APPDIR, which only the AppImage runtime sets.
+# seaf-cli also requires seaf-daemon on PATH, so keep usr/bin ahead of it.
+RUN printf '%s\n' \
+        '#!/bin/sh' \
+        'APPDIR=/opt/seafile-cli' \
+        'export PATH="$APPDIR/usr/bin:$PATH"' \
+        'export LD_LIBRARY_PATH="$APPDIR/usr/lib:$LD_LIBRARY_PATH"' \
+        'export PYTHONPATH="$APPDIR/usr/lib/python3.9/site-packages:$PYTHONPATH"' \
+        'exec python3 "$APPDIR/usr/bin/seaf-cli" "$@"' \
+        > /usr/local/bin/seaf-cli && \
+    chmod +x /usr/local/bin/seaf-cli
+
 # Use virtual environment
 ENV VIRTUAL_ENV=/opt/venv
 RUN python3 -m venv --system-site-packages $VIRTUAL_ENV
 ENV PATH="$VIRTUAL_ENV/bin:$PATH"
 
+# The APT package used to install the seafile/pysearpc RPC bindings system-wide.
+# The AppImage keeps them in its own tree, so put them on the venv path: dsc
+# imports seafile directly, not only through seaf-cli. They are pure Python, so
+# the 3.9 build works unchanged on this interpreter.
+RUN python3 -c "import pathlib, site; \
+    pathlib.Path(site.getsitepackages()[0], 'seafile-appimage.pth') \
+        .write_text('/opt/seafile-cli/usr/lib/python3.9/site-packages\n')"
+
 # Install app requirements
 WORKDIR /dsc
 COPY requirements.txt ./
@@ -32,5 +65,10 @@ RUN chmod +x /dsc/start.py && \
     mkdir -p /dsc/seafile-data && \
     chown seafile:seafile -R /dsc
 
+# Smoke checks: seaf-cli has no --version flag, so assert its subcommands, and
+# confirm the app's own imports resolve.
+RUN seaf-cli --help | grep -q 'download-by-name' && \
+    python3 -c "import seafile, pysearpc, dsc.client, dsc.misc"
+
 VOLUME /dsc/seafile-data
 CMD ["./start.py"]