`nginx:latest` is based on Debian Stretch and is listed as being 45MB,
while `nginx:alpine` is based on Alpine 3.9 and is just 7MB.
Basing the riot-web Docker image on `nginx:alpine` should
make it 38MB smaller compared to the current situation.
The nginx version seems to be "latest mainline" on both (currently 1.15.12),
judging by each Dockerfile:
- e5123eea0d/mainline/stretch/Dockerfile
- e5123eea0d/mainline/alpine/Dockerfile