docs(CONTRIBUTING.md): update steps for adding a new extension

[qiaofenlin]

Fix broken Dockerfile link in CONTRIBUTING.md

Description:
I noticed that the Dockerfile link in the CONTRIBUTING.md file was broken (404). I have attempted to fix this issue in this pull request.

Reason for Major Changes:
While fixing the link, I found that there are multiple versions of Dockerfiles in the current project. Therefore, I updated the documentation to reflect this and to help contributors add extensions to the appropriate Dockerfile(s).

Screenshots:

Notes:
If there are any problems with my changes, please feel free to point them out.
Or, feel free to make further modifications based on this PR. Thank you!

[samrose]

@qiaofenlin thanks for your great contribution! I left a few notes on on the PR. The contributing is very out of date. Definitely the change you submitted about supporting multiple Dockerfiles is good and accurate. The rest is superceded by Supabase use of Nix. I think we'll include your contribution on the Dockerfile variations, but we need to also update the rest on how to contribute extensions.

[qiaofenlin]

@samrose Thank you for your review and suggestions!

I have updated the documentation according to your feedback and addressed the issues you pointed out.
If there’s anything else that needs adjustment, please let me know. Thanks again for your help!

[samrose]

We should also remove all these lines since we don't do any of this in Dockerfiles any more

Here's a minimal example:

ARG pg_graphql_release=1.1.0

####################
# 19-pg_graphql.yml
####################
FROM base as pg_graphql
# Download package archive
ARG pg_graphql_release
ADD "https://github.com/supabase/pg_graphql/releases/download/v${pg_graphql_release}/pg_graphql-v${pg_graphql_release}-pg${postgresql_major}-${TARGETARCH}-linux-gnu.deb" \
    /tmp/pg_graphql.deb

####################
# Collect extension packages
####################
FROM scratch as extensions
COPY --from=pg_graphql /tmp/*.deb /tmp/

Using this process maximises the effectiveness of Docker layer caching, which significantly speeds up our CI builds.

[samrose]

If you remove those lines, I'll approve and we'll expand on it from there, as there is more to know, but I don't expect anyone outside of Supabse to yet know all of the other contributing info based on recent changes.