We were mounting our own configuration to
`/usr/share/nginx/html/config.json`, which is a symlink to
`/tmp/config.json`. So we effectively mount our file to
`/tmp/config.json`.

When starting:

- if Hydrogen sees a `CONFIG_OVERRIDE` environment variable,
  it will try to save it into our read-only config file and fail.

- if Hydrogen doesn't see a `CONFIG_OVERRIDE` environment variable (the
  path we go through, because we don't pass such a variable),
  it will try to copy its bundled configuration (`/config.json.bundled`)
  to `/tmp/config.json`. Because our configuration is mounted as read-only, it will
  fail.

In both cases, it will fail with:

> cp: can't create '/tmp/config.json': File exists

Source: 3720de36bb/docker/dynamic-config.sh

We work around this by mounting our configuration on top of the bundled
one (`/config.json.bundled`). We then let Hydrogen's startup script copy
it to `/tmp/config.json` (a tmpfs we've mounted into the container) and use it from there.