Fix chainspecs path in Dockerfile

[helvenk]

Description

I found a docker build error shows raw_spec_finney.json not exists, which make the docker container exited.

node-subtensor \
          --base-path /tmp/blockchain \
          # Here !!!
          --chain raw_spec_finney.json \
          --rpc-external --rpc-cors all \

This happens on Windows and Linux, but I didn't encounter on macOS. Perhaps it's because macOS automatically recursively searches subdirectories for JSON files? I don't know.

COPY --from=prod_builder /build/*.json /

This PR try to fix it.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[open-junius]

@helvenk Thanks for the PR. There is a PR #1561 already merged into devnet-ready. we just need to wait it to be in main branch.