Option for location of postgraphile.tags.json5

### Feature description

Currently the CLI searches for the `postgraphile.tags.json5` file in the working directory.
It would be good addition an option to specify a full path of a configuration file.

### Motivating example

The official docker image has the `/postgraphile` as the working directory.
What if you want to run a container from the official image, but provide your own configuration file.
You could add the configuration file to a volume, and point the execution to a specific path.
It's not possible to mount a volume and just change the container working directory (using `-w`), because the image command relies on the working directory having all the code.

### Breaking changes

The new option would be optional, default to the current behavior. So no breaking change.

### Supporting development

I [tick all that apply]:

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [x] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile