Add /usr/local/bundle/bin to the path in the cf-uaac image

In the latest Ruby alpine image they removed the /usr/local/bundle/bin path
from the PATH which causes installed gem binaries not to be on the path
anymore.

We need to temporarily fix this until they sort out
docker-library/ruby#209.

cf-uaac/Dockerfile

@@ -1,5 +1,8 @@
 FROM ruby:2.5-alpine
 
+# TODO: remove after https://github.com/docker-library/ruby/pull/209 was fixed.
+ENV PATH "/usr/local/bundle/bin:${PATH}"
+
 RUN apk add --update musl-dev gcc make g++ && rm -rf /var/cache/apk/*
 
 RUN gem install cf-uaac -v 3.2.0 --no-rdoc --no-ri