feat: clone-and-run docker compose for standalone Handle server

[MatusBeke]

What

Makes the repo clone & run: clone it, set one variable, docker compose up, and
the Handle server starts as a standalone service wired to a DSpace backend.

git clone --recurse-submodules https://github.com/dataquest-dev/docker-handle-server.git
cd docker-handle-server
cp .env.example .env          # set DSPACE_HANDLE_ENDPOINT to your backend (…/server)
docker compose up -d

The Handle server resolves through the backend's remote-resolver REST endpoints and
never touches the database directly:

client -> handle-server(:8000) --HTTP /server/resolve--> DSpace backend --> database

Changes

File	Change
docker-compose.yml	new — single-service compose; writes the plugin endpoint from DSPACE_HANDLE_ENDPOINT, waits for the backend, then starts the Handle server
.env.example	new — backend endpoint + published ports
assets/config/config.dct	default to the remote-resolver storage plugin (storage_class = MultiRemoteDSpaceRepositoryHandlePlugin) so resolution works out of the box
.gitattributes	force LF on the extensionless hdl* launchers (they were checked out as CRLF on Windows, breaking the shebang → docker build failed with hdl-server: not found, exit 127)
Readme.md	document the clone-and-run flow

Backend requirement

The backend must expose the remote-resolver endpoints:

handle.remote-resolver.enabled = true

Tested

• docker compose up -d against a running DSpace 7.5 backend: the Handle server loaded
  all prefixes from the backend and resolved handles (responseCode: 1).
• Verified in real time — inserting a fresh handle into Postgres made it immediately
  resolvable through the standalone Handle server.

Notes / follow-ups (not in this PR)

• Per-deployment keys should be generated with hdl-setup-server (the committed
  assets/config/*.bin are placeholders).
• Customer prefix + handle.net registration are deployment steps.

[MatusBeke]

Closing — not needed. The Handle-server separation is delivered by dataquest-dev/dspace-angular#1455, which works with the current published image (it patches the resolver config at container start). Changes to this image repo were not requested.