Allow not retaining the path, do not add trailing slashes and update nginx

[leMaik]

• Update nginx to 1.25 (latest version at the time of writing)
• Do not append trailing slashes to the redirect target (fixes Remove trailing slash on the redirect target #10)
• Add RETAIN_PATH env variable that can be set to false to not retain the path, ie. to redirect any path to the same target (defaults to true to keep the old behavior)

Unlike eg. #4, this PR should not break existing setups.