With the following Dockerfile
FROM fpco/stack-build:lts-17.12 as dependencies
ENV STACK_ROOT /home/stackage/.stack
COPY ./stack.yaml ./package.yaml ./
RUN stack build --dependencies-only
Stack will always start by downloading and installing GHC from scratch before building.
> [dependencies 5/7] RUN stack build --dependencies-only 2.8s
=> => # Preparing to install GHC to an isolated location.
=> => # This will not interfere with any system-level installation.
=> => # Preparing to download ghc-8.10.4 ...
=> => # ghc-8.10.4: download has begun
=> => # ...
I've tried different values for the STACK_ROOT environment variable (including none, leaving the default) but none of it worked.
Now, if I comment out the RUN stack build line and just pull the image, then run a shell into it, I can see that GHC and Stack are already pre-installed in /home/stackage/.stack, with the Linux binaries.
How comes Stack always chooses to download and install everything from scratch every time despite this? Is there a way to avoid it?
EDIT: this answer seemed to indicate that using STACK_ROOT=/home/stackage/.stack would work, but apparently it doesn't — maybe I'm missing something?