Lock bundler to a specific version

There were some changes in bundler 2.0.2 to 2.1.0 and official ruby
images that was causing issue with ruby gems executables. This is what
causing our build process to fail.

There are some fixes coming up in ruby 2.7 image and next bundler
release, but for now let's lock the bundler version to 2.0.0.

Related: rubygems/bundler#7494
Fixes: #46

metanorma/Dockerfile.in

@@ -13,7 +13,7 @@ RUN pip install --upgrade pip
 RUN pip install idnits xml2rfc --ignore-installed six chardet
 
 # install latest bundler
-RUN gem install bundler
+RUN gem install bundler -v "2.0.2"
 
 # install metanorma toolchain
 RUN mkdir -p /setup