docker/golang: fix brotli multi-stage build — empty submodule + cmake version

[Copilot]

Two cascading build failures in the brotli-builder stage:

1. submodules/brotli is an uninitialised git submodule (empty dir), so COPY copied nothing and cmake errored with "does not appear to contain CMakeLists.txt"
2. After fixing the source, ubuntu:bionic's system cmake (3.10.2) is too old — brotli at the pinned commit requires ≥ 3.15

Changes

• Source acquisition: replace COPY submodules/brotli with git clone + git checkout at the pinned submodule commit (664952333), making the builder self-contained
• CMake version: drop system cmake package from builder's apt deps; install CMake 3.31.0 from Kitware (same tarball the final stage already uses)
• Supply-chain hardening: add SHA256 verification (sha256sum -c) to both cmake downloads — previously neither was verified

FROM ubuntu:bionic AS brotli-builder
RUN apt-get install build-essential git wget

RUN wget https://github.com/Kitware/CMake/releases/download/v3.31.0/cmake-3.31.0-linux-x86_64.tar.gz && \
    echo "0fcb338b...  cmake-3.31.0-linux-x86_64.tar.gz" | sha256sum -c && \
    tar xf cmake-3.31.0-linux-x86_64.tar.gz -C /usr --strip-components=1

RUN git clone https://github.com/google/brotli.git /tmp/brotli && \
    git -C /tmp/brotli checkout 664952333f675eb189b9d8dfefb1d33538cc41f6

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE] </issue_title>
<issue_description>### What feature would you like?

надо вот эту команду https://github.com/chipitsine/wal-g/blob/master/docker/golang/Dockerfile#L29-L32 преобразовать в multi stage build, чтобы исходная папка была в временном контейнере, а в образ вошел только результат

Why is this feature useful?

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [FEATURE] <what you want> #52

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