Tweak Dockerfile (#6265)

* Don't invalidate cache when Dockerfiles themselves change

* Remove grpc sources (frees up 1.7GB)

Author: ErikSchierboom

.dockerignore

@@ -12,3 +12,4 @@ storage/
 test/
 tmp/
 vendor/
+docker/*.Dockerfile

docker/rails.Dockerfile

@@ -28,7 +28,10 @@ RUN gem install nokogiri -v 1.14.2 && \
 COPY Gemfile Gemfile.lock ./
 RUN bundle config set deployment 'true' && \
     bundle config set without 'development test' && \
-    bundle install
+    bundle install && \
+    grpc_path="$(bundle show --paths grpc)/src/ruby/ext/grpc" && \
+    make -C "${grpc_path}" clean && \
+    rm -rf "${grpc_path}/libs" "${grpc_path}/objs"
 
 # Only package.json and yarn.lock changes require a new yarn install
 COPY package.json yarn.lock ./