fix(docker): warn instead of rejecting default or missing credentials (#4728)

* fix(docker): warn instead of rejecting default or missing credentials

The entrypoint hard-reject from #4278 broke the container-first UX and
the repo's own scheduled e2e lanes (e2e-s3tests, mint boot rustfs-ci
images with default credentials and died at startup). Maintainer
decision: ship no baked-in credentials, warn instead of block.

- missing credentials: warn and start; wording accounts for the
  RUSTFS_ROOT_*/MINIO_* alias sources the entrypoint does not inspect
- default rustfsadmin via env/CLI/file: warn and start; the warning
  notes all-default pairs cannot derive an internode RPC secret
- malformed config stays fatal: source conflicts, unreadable files,
  empty or whitespace-only values, flags missing their argument
- present-but-empty env vars now hit the empty-value hard failure
  instead of running the binary with an empty root credential
- empty/default checks trim CR and blanks like the binary; files
  without a trailing newline are no longer falsely rejected as empty
- the no-baked-credentials guard covers all four Dockerfiles, and the
  test harness refuses hosts where /usr/bin/rustfs exists
- e2e-s3tests/mint move to non-default credentials (rustfsadmin-ci),
  which also restores RPC-secret derivation for the multi-node lane

GHSA-68cw fail-closed RPC derivation (#4402) is untouched; helm stays
fail-closed by design.

* chore(docker): reword entrypoint comment flagged by typos check

.github/workflows/e2e-s3tests.yml

@@ -85,8 +85,8 @@ on:
 
 env:
   # main user
-  S3_ACCESS_KEY: rustfsadmin
-  S3_SECRET_KEY: rustfsadmin
+  S3_ACCESS_KEY: rustfsadmin-ci
+  S3_SECRET_KEY: rustfssecret-ci
   # alt user (must be different from main for many s3-tests)
   S3_ALT_ACCESS_KEY: rustfsalt
   S3_ALT_SECRET_KEY: rustfsalt

.github/workflows/mint.yml

@@ -72,8 +72,8 @@ on:
     - cron: "0 6 * * 0"
 
 env:
-  S3_ACCESS_KEY: rustfsadmin
-  S3_SECRET_KEY: rustfsadmin
+  S3_ACCESS_KEY: rustfsadmin-ci
+  S3_SECRET_KEY: rustfssecret-ci
   S3_REGION: us-east-1
 
   RUST_LOG: info