CI: fix docker build

Dockerfile

@@ -25,7 +25,8 @@ RUN if $GUI; then \
 RUN ln -s /usr/bin/python3 /usr/local/bin/python && ln -s /usr/bin/pip3 /usr/local/bin/pip
 # NumPy version is conflicting with system's gdal dep and may require venv
 ARG NUMPY_SPEC="==1.22.*"
-RUN pip install --no-cache-dir -U pip wheel mock six future tqdm deprecated "numpy$NUMPY_SPEC" \
+ARG PROTO_SPEC="==3.20.*"
+RUN pip install --no-cache-dir -U pip wheel mock six future tqdm deprecated "numpy$NUMPY_SPEC" "protobuf$PROTO_SPEC" \
  && pip install --no-cache-dir --no-deps keras_applications keras_preprocessing
 
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