docker-composeでrailsの環境構築時に出るエラーについて

下記のようなエラーに出くわしたので備忘録。

mbp:sample_app kentaroyoshizumi$ docker-compose run web rails new . --force --database=mysql --skip-bundle
Pulling db (mysql:8.0)...
8.0: Pulling from library/mysql
4be315f6562f: Already exists
96e2eb237a1b: Already exists
8aa3ac85066b: Already exists
ac7e524f6c89: Already exists
f6a88631064f: Already exists
15bb3ec3ff50: Already exists
ae65dc337dcb: Already exists
573c3c7fa18d: Pull complete
9d10771b98b8: Pull complete
3d8ef442614b: Pull complete
7dc17a6cea26: Pull complete
752752efdaea: Pull complete
Digest: sha256:2dafe3f044f140ec6c07716d34f0b317b98f8e251435abd347951699f7aa3904
Status: Downloaded newer image for mysql:8.0
Building web

Traceback (most recent call last):
  File "compose/cli/main.py", line 81, in main
  File "compose/cli/main.py", line 203, in perform_command
  File "compose/metrics/decorator.py", line 18, in wrapper
  File "compose/cli/main.py", line 968, in run
  File "compose/cli/main.py", line 1426, in run_one_off_container
  File "compose/project.py", line 664, in up
  File "compose/service.py", line 364, in ensure_image_exists
  File "compose/service.py", line 1111, in build
  File "compose/service.py", line 1841, in build
compose.service.BuildError: (<Service: web>, {'message': 'dockerfile parse error line 4: unknown instruction: //'})

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "docker-compose", line 3, in <module>
  File "compose/cli/main.py", line 92, in main
TypeError: can only concatenate str (not "dict") to str
[93647] Failed to execute script docker-compose

結論Dockerfile内のコマンドの記述が誤っていた。

FROM ruby:2.5
RUN apt-get update -qq && \           //←バックスラッシュが抜けていたり、コメントアウトがきいていなかったりする。
  apt-get install -y build-essential libpq-dev nodejs

RUN mkdir /myapp
WORKDIR /myapp

COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock

RUN bundle install
COPY . /myapp

ここを修正してうまく構築できた。