[build] Use tools with stable version in building docs

docs/Dockerfile

@@ -1,6 +1,6 @@
 FROM python:3.7-slim
 RUN apt-get update
 RUN apt-get -y install git
-RUN pip3 install -U sphinx==4.1.1 myst-parser pygments sphinx-rtd-theme sphinx-autobuild gitpython
+RUN pip3 install -U sphinx==4.1.1 myst-parser==0.15.2 pygments==2.10.0 sphinx-rtd-theme==0.5.2 sphinx-autobuild==2021.3.14 gitpython==3.1.18
 EXPOSE 8001
 CMD ["sphinx-autobuild", "--host", "0.0.0.0", "--port", "8001", "/home/flink-cdc/docs", "/home/flink-cdc/docs/_build/html"]

docs/build_docs.sh

@@ -22,7 +22,7 @@ set -x
 # step-1: install dependencies
 apt-get update
 apt-get -y install git rsync python3-pip python3-git python3-stemmer python3-virtualenv python3-setuptools
-python3 -m pip install -U sphinx==4.1.1 myst-parser pygments sphinx-rtd-theme
+python3 -m pip install -U sphinx==4.1.1 myst-parser==0.15.2 pygments==2.10.0 sphinx-rtd-theme==0.5.2
 
 export SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct)
 export REPO_NAME="${GITHUB_REPOSITORY##*/}"