我的多阶段构建/assets/css在复制目录时找不到目录。我需要更改什么？

Docker版本

版本17.06.0-ce-mac17（18432）

通道：边缘

4bb7a7dfa0

myimage：sass我在多阶段构建中使用的图像

FROM ruby:2.4.1-slim

RUN mkdir /assets
VOLUME /assets

RUN gem install sass
ENTRYPOINT ["sass"]

多阶段构建Dockerfile

注意我在第一个图像上运行的调试命令cd /assets/css && ls && pwd，结果在构建阶段中显示。

# Compile Sass
FROM myimage/sass AS builder
COPY app/assets/sass /assets/sass
RUN sass --update --force --sourcemap=none --stop-on-error /assets/sass:/assets/css &&\
  # sass directory isn't needed
  rm -r assets/sass &&\
  # debugging: check /assets/css exists inside the container
  cd /assets/css && ls && pwd

FROM alpine:3.6
WORKDIR /app
RUN mkdir /app/logs
VOLUME ["/app/logs"]
COPY --from=builder /assets/css /app/assets/css
EXPOSE 80
CMD ["./bin/bash"]
Raw

输出 docker build -t myimage:css .

请注意cd /assets/css && ls && pwd步骤3/11的输出，显示该/assets/css目录存在并且其中包含一个main.css文件

Step 1/11 : FROM myimage:sass AS builder
 ---> 7c6662186d55
Step 2/11 : COPY app/assets/sass /assets/sass
 ---> 76b5d86846b8
Removing intermediate container ee74d16617b4
Step 3/11 : RUN sass --update --force --sourcemap=none --stop-on-error /assets/sass:/assets/css &&  rm -r assets/sass &&  cd /assets/css && pwd
 ---> Running in 83dc591edc5c
  directory /assets/css
      write /assets/css/main.css
main.css
/assets/css
 ---> 3939f46fb355
Removing intermediate container 83dc591edc5c
Step 4/11 : FROM alpine:3.6
 ---> 7328f6f8b418
Step 5/11 : WORKDIR /app
 ---> 19ad596f9fc1
Removing intermediate container 790fac2040f1
Step 6/11 : RUN mkdir /app/logs
 ---> Running in cb66151a4694
 ---> 18d6c4970d04
Removing intermediate container cb66151a4694
Step 7/11 : VOLUME /app/logs
 ---> Running in b8a98a38a054
 ---> fa68603ccf30
Removing intermediate container b8a98a38a054
Step 8/11 : COPY --from=builder /assets/css /app/assets/css
COPY failed: stat /var/lib/docker/aufs/mnt/0ddcc250ed9c4eb1de46305e62cb2303274b027b2c9b0ddd09471fce3c3ed619/assets/css: no such file or directory

那么为什么不能/assets/css复制到最终图像中呢？Docker引擎为什么找不到它？