Fix error with rubygems-update

```
ERROR:  Error installing rubygems-update:
	There are no versions of rubygems-update (= 3.4.3) compatible with your Ruby & RubyGems
	rubygems-update requires Ruby version >= 2.6.0. The current ruby version is 2.5.9.229.
ERROR:  While executing gem ... (Errno::ENOENT)
    No such file or directory @ dir_chdir - /usr/local/bundle/gems/rubygems-update-3.0.3
```

Same issue as rubygems/rubygems#2534,
following the suggested workaround in the comments there to install an
older version of rubygems-update

rails/Dockerfile

@@ -17,7 +17,7 @@ RUN apk --no-cache add --update \
  libc6-compat \
  imagemagick \
  ffmpeg && \
- gem update --system --no-document && \
+ gem install rubygems-update -v '<3' --no-document && update_rubygems && \
  gem install bundler --no-document --force
 
 WORKDIR /api