Using the `envsubst` command, we can replace environment variables in
the nginx template files. This allows for more flexibility when
configuring the nginx server.
The Docker image we use as base for Nginx does provide the
`20-envsubst-on-templates.sh` script that will replace environment
variables in the template files.
This change does not include any behavior change, but unblocks future
changes that require environment variables in the nginx configuration.
This change allows setting environment variables with a `_FILE` suffix,
which will be used to load the contents of the file specified in the
variable into the variable without the suffix.
For example, setting `ROMM_AUTH_SECRET_KEY_FILE=/run/secrets/romm_auth_secret_key`
and creating a file with the secret key at the specified path will set
`ROMM_AUTH_SECRET_KEY` to the contents of the file.
A common use case for this is to use secrets in Docker Compose [1], to
avoid exposing secrets in the `docker-compose.yml` or `env` files.
[1] https://docs.docker.com/compose/how-tos/use-secrets/
