@dutsnekcirf
Basically there is no need to run a website within a subdirectory behind a reverse proxy. This makes things more complicated.
However, HAproxy is able to insert a string at the beginning of the path. You can use "http-request set-path" to do this.
You can set it in the frontend or backend. The preferred method depends on your setup.
Add an action, select "http-request set-path" and enter "/web/%[path]" below. This assumes that the website has further subdirectories.
However, with this, the additional path is inserted into all requests.If your website send URLs to call to the client, which already inlcludes the "/web/", you have to bind this action to an ACL to ensure it is not applied in this case.