nokogiri: use bundled libxml2.

Make nokogiri use the bundled version of libxml2 instead of the system
version. In the past installing nokogiri was slow because it had to
compile the bundled version of libxml2, which is partly why we switched
to the system library. Now it's faster because the bundled version comes
pre-compiled with the nokogiri gem.

https://nokogiri.org/#native-gems-faster-more-reliable-installation

Reverts 440bbbb28.

config/docker/Dockerfile.danbooru

@@ -26,7 +26,7 @@ ARG NODE_VERSION
 RUN \
   apt-get update && \
   DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-    build-essential libpq-dev libvips-dev libxml2-dev libxslt-dev zlib1g-dev \
+    build-essential libpq-dev libvips-dev zlib1g-dev \
     curl ca-certificates dirmngr git gpg && \
   rm -rf /var/lib/apt/lists/*
 
@@ -36,7 +36,7 @@ RUN \
   asdf global nodejs "$NODE_VERSION" && \
   npm install --unsafe-perm=true -g yarn
 
-COPY .bundle Gemfile Gemfile.lock ./
+COPY Gemfile Gemfile.lock ./
 RUN \
   bundle install --jobs "$(nproc)" && \
   rm -rf "$(gem environment gemdir)/cache" && \