fix: respect executablePath in standalone mode

[cvetkovski98]

The docs and types both allow { standalone: true, executablePath: '/path/to/docker-compose' }, but the runtime condition executable?.standalone && !executable.executablePath causes this combination to fall into the non-standalone branch, which appends compose as a subcommand, producing an invalid command like /path/to/docker-compose compose up.

We hit this using testcontainers within a hermetic Bazel environment and a docker-compose binary managed by Bazel. The binary lives at a known absolute path (not on PATH), so standalone: true alone doesn't work, it only searches for docker-compose on PATH. Setting both standalone and executablePath seemed like the obvious solution based on the docs, but silently fell through to the wrong code path.

Added new unit tests verifying:

• { standalone: true, executablePath } uses the custom path without compose
• { standalone: true } defaults to docker-compose
• { executablePath } (non-standalone) still appends compose (regression test for unchanged behavior)

[AlexZeitler]

@cvetkovski98 Thanks for the PR - new version is available on npm.

[cvetkovski98]

@AlexZeitler Thanks for the quick review and merge, appreciated!