Makefile: fix docker-check/docker-indent from a git worktree

[dimitri]

Summary

make docker-check/make docker-indent (the documented way to run
citus_indent locally without installing it) failed when run from a git
worktree:

fatal: not a git repository: <main-repo>/.git/worktrees/<name>
fatal: not a git repository: <main-repo>/.git/worktrees/<name>
could not list files under source control

citus_indent calls git ls-files to find which files to check. A
worktree's own .git is just a pointer file (gitdir: <main-repo>/.git/ worktrees/<name>) to a path outside the container's mounted volume, so
that call fails inside the container.

Fix

Resolve the real common git dir on the host via git rev-parse --git-common-dir and bind-mount it into the container at the identical
absolute path, so the worktree's pointer still resolves. This is a no-op
for a plain (non-worktree) checkout, where the common dir already lives
under the mounted source tree — verified both cases work.

Also documents the make docker-check/make docker-indent shortcut in
CONTRIBUTING.md, which previously only described installing citus_indent
locally.

Verification

• make docker-check from both a plain checkout and a git worktree
  checkout — both pass (exit 0).
• make docker-indent — no unexpected reformatting.
• ci/banned.h.sh — unaffected, still passes.