feat(supabase): make storage S3 backend configurable via env

[Carlos-Vera]

Problem

The Supabase template ships the S3 backend settings as commented-out literals:

      STORAGE_BACKEND: file
      #GLOBAL_S3_ENDPOINT: https://your-s3-endpoint
      #GLOBAL_S3_PROTOCOL: https
      #GLOBAL_S3_FORCE_PATH_STYLE: "true"
      #AWS_ACCESS_KEY_ID: your-access-key-id
      #AWS_SECRET_ACCESS_KEY: your-secret-access-key

So pointing storage at an external S3-compatible service means editing the compose file
by hand. On platforms like EasyPanel the service is sourced from git and the file is
rewritten on every deploy, so the edit is lost. And since no service in this template
uses env_file, only what the YAML declares under environment: ever reaches the
container — setting these in the platform's env UI has no effect.

docker-compose.s3.yml does not cover this case: it wires up a local MinIO with a fixed
GLOBAL_S3_ENDPOINT: http://minio:9000.

Change

Expose the five settings as environment variables, keeping file as the default so
existing deployments are unaffected, and document them in .env.example.

Also adds TUS_ALLOW_S3_TAGS (default true, unchanged behaviour): Cloudflare R2 does
not implement x-amz-tagging, and resumable (TUS) uploads fail with HTTP 500 unless it
is set to "false".

Verification

$ docker compose --env-file .env.example config | grep -E '^ +STORAGE_BACKEND:'
      STORAGE_BACKEND: file

$ STORAGE_BACKEND=s3 GLOBAL_S3_ENDPOINT=https://acct.r2.cloudflarestorage.com \
  TUS_ALLOW_S3_TAGS=false AWS_ACCESS_KEY_ID=k AWS_SECRET_ACCESS_KEY=s \
  docker compose --env-file .env.example config | grep -E 'STORAGE_BACKEND|GLOBAL_S3_ENDPOINT|TUS_ALLOW_S3_TAGS'
      GLOBAL_S3_ENDPOINT: https://acct.r2.cloudflarestorage.com
      STORAGE_BACKEND: s3
      TUS_ALLOW_S3_TAGS: "false"

Default deployments resolve exactly as before; only an explicit STORAGE_BACKEND=s3
changes anything.

Running in production on a Supabase self-host serving ~29k images from Cloudflare R2.

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By Carlos Vera of BravesLab
con el favor de nuestro señor JesusCristo.