fix gitpod covers

[RayBB]

Closes #5834

Technical

A pretty simple fix at the end of the day, just injecting the right domain in to the config in the gitpod setup

Testing

1. Start a gitpod of this PR at https://gitpod.io/#https://github.com/internetarchive/openlibrary/pull/9869
2. Upload an image
3. See the image works

Screenshot

Stakeholders

[cdrini]

Backup:

    # Replace coverstore_public_url with the URL of the GitPod port
    sed -i "s|coverstore_public_url: .*|coverstore_public_url: $(gp url 7075)|" conf/openlibrary.yml

[RayBB]

@cdrini I think I came up with a decent solution :)

When the env/web.env file is missing nothing changes.
When it exists it's used.
env/ was already in the .gitignore so that's where I chose to put it.
Now we have a nice way to override env variables without changing the yaml :)

[cdrini]

Nice lgtm! Ray confirmed this works in gitpod!

(Side note: I like this strategy quite a bit... in some universe it might be nice to move all our openlibrary.yml configs to environment variables, it would make for significantly easier configuring! CC @scottbarnes )