precommit: Introduce new server-based framework

tools/precommit/Dockerfile

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+FROM ubuntu:20.04
+
+# author: Ole Schuett
+
+# Install Ubuntu packages.
+RUN export DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true && \
+    apt-get update && apt-get install -y --no-install-recommends \
+    clang-format \
+    git \
+    less \
+    python3 \
+    python3-pip \
+    python3-wheel \
+    python3-setuptools \
+    rubygems \
+    shellcheck \
+    vim \
+  && rm -rf /var/lib/apt/lists/*
+
+# Install Markdownlint.
+RUN gem install mdl
+
+# Install Black Python Formatter.
+RUN pip3 install black
+
+# Clone cp2k repository (needed for CI mode).
+RUN git clone --quiet --recursive --single-branch -b master https://github.com/cp2k/cp2k.git /workspace/cp2k
+
+# Install Flask app.
+RUN pip3 install flask gunicorn
+WORKDIR /opt/cp2k-precommit
+COPY precommit_server.py .
+
+ARG REVISION
+ENV REVISION=${REVISION}
+
+COPY entrypoint.sh .
+CMD ["./entrypoint.sh"]
+
+#EOF