Error when generating config with docker mounted config dir

### Description

Mounting a directory from the host to use as the config dir fails to generate a config.

### Steps to reproduce

```sh
mkdir conf
podman run --rm --name synapse \
--mount type=volume,src=synapse-data,dst=/data \
--mount type=bind,src=./conf,dst=/conf \
-e SYNAPSE_SERVER_NAME=matrix.example.com \
-e SYNAPSE_REPORT_STATS=yes \
-e SYNAPSE_CONFIG_DIR=/conf \
-e SYNAPSE_ENABLE_METRICS=1 \
docker.io/matrixdotorg/synapse:v1.147.0 generate
```
This crashes with the following traceback:
```
Traceback (most recent call last):
File "/start.py", line 321, in
main(sys.argv, os.environ)
~~~~^^^^^^^^^^^^^^^^^^^^^^
File "/start.py", line 251, in main
return run_generate_config(environ, ownership)
File "/start.py", line 200, in run_generate_config
convert("/conf/log.config", log_config_file, environ)
~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/start.py", line 61, in convert
with open(src) as infile:
~~~~^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/conf/log.config'
```

### Homeserver

N/A

### Synapse Version

1.147

### Installation Method

Docker (matrixdotorg/synapse)

### Database

N/A

### Workers

Single process

### Platform

KDE Neon 24.04 running on bare metal

### Configuration

_No response_

### Relevant log output

```shell
See repro steps.
```

### Anything else that would be useful to know?

_No response_