Fix accidental prebuilt PyAV usage in the Dockerfile (#6573)

When I integrated pip-compile, I neglected to notice that in the
generated `*.txt` files the --no-binary=av option is on its own line,
and therefore is stripped away by the sed command.

Hardcode this option in the Dockerfile to make sure we build PyAV from
source. Also, add a workaround for the fact that PyAV is incompatible
with the recently-released Cython 3.

CHANGELOG.md

@@ -30,7 +30,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
-- TDB
+- Accidentally using prebuilt FFmpeg bundled in PyAV instead of the custom
+  build.
 
 ### Security
 

Dockerfile

@@ -56,8 +56,11 @@ COPY utils/dataset_manifest/requirements.txt /tmp/utils/dataset_manifest/require
 RUN grep -q '^av==' /tmp/utils/dataset_manifest/requirements.txt
 RUN sed -i '/^av==/!d' /tmp/utils/dataset_manifest/requirements.txt
 
+# Work around https://github.com/PyAV-Org/PyAV/issues/1140
+RUN pip install setuptools wheel 'cython<3'
+
 RUN --mount=type=cache,target=/root/.cache/pip/http \
-    python3 -m pip wheel \
+    python3 -m pip wheel --no-binary=av --no-build-isolation \
     -r /tmp/utils/dataset_manifest/requirements.txt \
     -w /tmp/wheelhouse