How do I copy variables between stages of multi stage Docker build?

You got 3 options:
The “ARG” solution, the “base” solution, and “file” solution.

ARG version_default=v1

FROM alpine:latest as base1
ARG version_default
ENV version=$version_default
RUN echo ${version}
RUN echo ${version_default}

FROM alpine:latest as base2
ARG version_default
RUN echo ${version_default}

another way is to use base container for multiple stages:

FROM alpine:latest as base
ARG version_default
ENV version=$version_default

FROM base
RUN echo ${version}

FROM base
RUN echo ${version}

You can find more details here:
https://github.com/moby/moby/issues/37345

Also you could save the hash into a file in the first stage, and copy the file in the second stage and then read it and use it there.