docker: use less image layers and update compose version

https://stackoverflow.com/questions/30256386/how-to-copy-multiple-files-in-one-layer-using-a-dockerfile

https://docs.docker.com/compose/compose-file/compose-file-v3/

Dockerfile

@@ -6,11 +6,7 @@ RUN apt-get update && apt-get install --no-install-recommends -y python3-pip she
 
 WORKDIR /app
 
-COPY Gemfile .
-
-COPY Gemfile.lock .
-
-COPY .pre-commit-config.yaml .
+COPY Gemfile Gemfile.lock .pre-commit-config.yaml ./
 
 RUN bundle install && pip3 install pre-commit && git init . && pre-commit install-hooks
 

docker-compose.yml

@@ -1,4 +1,4 @@
-version: '3'
+version: '3.8'
 services:
   test:
     build: