Docker build stage for JXL

Dockerfile

#...
WORKDIR /rails
#...

# https://github.com/libjxl/libjxl/blob/main/BUILDING.md
FROM base as buildjxl
ENV CC=clang
ENV CXX=clang++
RUN apt-get update -qq && \
    apt-get install --no-install-recommends -y git cmake pkg-config libbrotli-dev libgif-dev libjpeg-dev libopenexr-dev libpng-dev libwebp-dev clang make build-essential && \
    git clone https://github.com/libjxl/libjxl.git --recursive --shallow-submodules && \
    cd libjxl && git submodule update --init --recursive --depth 1 --recommend-shallow
RUN cd libjxl && mkdir build && cd build && \
    cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTING=OFF .. && \
    cmake --build . -- -j$(nproc) && ls -l tools
    
# Sometime later in the final stage....

COPY --from=buildjxl /rails/libjxl/build/tools/cjxl bin/
COPY --from=buildjxl /rails/libjxl/build/tools/djxl bin/
COPY --from=buildjxl /rails/libjxl/build/lib /rails/

# The final layer will need `libgif7` installed.