Docker中的Haskell Stack：额外的功能不起作用

Question

我尝试使用此Dockerfile在Docker容器中编译程序：

FROM haskell:8.4

COPY stack.yaml   /opt/waziup/stack.yaml
COPY foo.cabal /opt/waziup/foo.cabal

WORKDIR /opt/waziup
RUN stack setup
RUN stack install --only-dependencies

COPY .  /opt/waziup
RUN stack install

我的stack.yaml文件是这样的：

resolver: lts-12.20
extra-deps:
 - servant-flatten-0.2
packages:
 - '.'

当我键入stack install时，此方法有效。但是，当我尝试编译docker时：

$ docker build .

servant-flatten-0.2: download
servant-flatten-0.2: configure
servant-flatten-0.2: build
servant-flatten-0.2: copy/register

...
    /opt/waziup/src/Foo.hs:37:1: error:
        Could not find module ‘Servant.API.Flatten’
        There are files missing in the ‘servant-flatten-0.2’ package,
        try running 'ghc-pkg check'.
        Use -v to see a list of the files searched for.

有什么主意吗？ Extra-deps软件包会发生什么？