I am using Odoo inside a Docker container behind an nginx reverse proxy, and have recently discovered that Odoo is unable to retrieve the daily currency rates from the ECB. When trying to update the rates manually, it gives me this error:

Unable to connect to the online exchange rate platform. The web service may be temporary down. Please try again in a moment.

The site in question that Odoo tries to access is this one:
http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml

However, it seems that I can access other websites from within Odoo just fine -- one example being the landing page of the Odoo App Store (In developer mode: Apps > App Store).

I have also tried to launch other containers within the same Docker network and tried to access the currency rate page of the ECB via curl etc. and succeeded.

Is there any extra configuration of Odoo's Docker container that I might have missed?