Revert "Don't copy ruby version file into the updater image (#7802)" (#…

…7835)

This reverts commit a89b3ac.

Turns out this is also used by rubocop in order to decide whether to
apply cops that depend on specific ruby versions or not. Without this,
running RuboCop inside a development shell does not run cleanly.

Dockerfile.updater-core

@@ -57,6 +57,7 @@ ENV GIT_LFS_SKIP_SMUDGE=1
 ARG SHIM="https://github.com/dependabot/git-shim/releases/download/v1.4.0/git-v1.4.0-linux-amd64.tar.gz"
 RUN curl -sL $SHIM -o git-shim.tar.gz && mkdir -p ~/bin && tar -xvf git-shim.tar.gz -C ~/bin && rm git-shim.tar.gz
 
+COPY --chown=dependabot:dependabot .ruby-version .ruby-version
 COPY --chown=dependabot:dependabot omnibus omnibus
 COPY --chown=dependabot:dependabot updater/Gemfile updater/Gemfile.lock dependabot-updater/