Update composer Docker tag to v2.8.9 (#12)

* Update composer Docker tag to v2.8.9

* Switch to minor version of composer instead of patch

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Co-authored-by: text-html-renovate[bot] <182508163+text-html-renovate[bot]@users.noreply.github.com>
Co-authored-by: Mathieu Rochette <mathieu@texthtml.net>

Author: text-html-renovate[bot]

.docker/Dockerfile

@@ -11,7 +11,7 @@ ARG USER_GID=${USER_UID}
 RUN groupadd --gid ${USER_GID} ${USERNAME} && \
     useradd -s /bin/bash --uid ${USER_UID} --gid ${USERNAME} -m ${USERNAME}
 
-COPY --from=composer:2.8.8 /usr/bin/composer /usr/bin/composer
+COPY --from=composer:2.8 /usr/bin/composer /usr/bin/composer
 
 RUN apt-get update && export DEBIAN_FRONTEND=noninteractive && \
     bash /tmp/library-scripts/dev.sh "${USERNAME}"