Base Docker image on nginx:alpine, not the larger nginx:latest

`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

Dockerfile

@@ -23,7 +23,7 @@ RUN cp /src/config.sample.json /src/webapp/config.json
 
 
 # App
-FROM nginx:latest
+FROM nginx:alpine
 
 COPY --from=builder /src/webapp /app