Update local Dockerfile used for spec changes to be in line with gi…

…thub workflows (#19718)

Currently the local server for the spec fails to build with the same
issue as in #19288 on a fresh
docker build.

This is the minimum amount of changes to actually get the spec building
for users to make changes. I highly recommend updating all the versions
when #19716 is finished.

docs/_spec/Dockerfile

@@ -7,9 +7,11 @@ RUN apt-get install -y curl \
   && apt-get install -y nodejs \
   && curl -L https://www.npmjs.com/install.sh | sh
 
-RUN gem update --system
+RUN gem install "rubygems-update:<3.5" --no-document
+RUN update_rubygems
+
 RUN gem install sass-embedded -v 1.58.0
-RUN gem install bundler:1.17.2 jekyll
+RUN gem install bundler:2.3.5
 
 WORKDIR /srv/jekyll
 

docs/_spec/Gemfile

@@ -1,5 +1,6 @@
 # To build the spec on Travis CI
 source "https://rubygems.org"
+ruby "~> 2.7"
 
 gem "jekyll", "3.6.3"
 gem "webrick"