Add volumes input for mounting host directories into the Firebird container

[mariuz]

Users needed a way to mount host filesystem paths into the Firebird Docker container to access .fdb files directly from the host — previously only achievable by running Docker manually.

Changes

• action.yml — New volumes input accepting newline-separated host_path:container_path mappings
• entrypoint.sh — Parses INPUT_VOLUMES and appends --volume flags to the docker run invocation
• testing.sh — Added volume test loop: starts container with a mapped volume, queries the DB, asserts the .fdb file is visible on the host
• testing_changes.yml — New ci-volumes job testing volume mounting across Firebird versions latest, 5, 4, 3; verifies the database file lands on the host path
• README.md — Documents the new parameter and adds it to the usage example

Usage

- uses: juarezr/firebirdsql-github-action@v2
  with:
    version: 'latest'
    firebird_database: 'my_database.fdb'
    firebird_user: 'my_user'
    firebird_password: 'my_password'
    volumes: '/tmp/firebird-data:/var/lib/firebird/data'

The host directory must exist and be writable before the action runs (e.g. sudo mkdir -p /tmp/firebird-data && sudo chmod 777 /tmp/firebird-data). Multiple volumes are supported — one per line.

[mariuz]

related issue #10