dockerfile: Install su-exec without updating. (#5051)

* Using --no-cache instead prevents unnecessarily
  adding about 1.3MB of Alpine package data
* This reduces the uncompressed image size with about 6%.

Dockerfile

@@ -18,8 +18,7 @@ RUN apk add --no-cache ca-certificates
 
 COPY --from=builder /go/src/github.com/syncthing/syncthing/syncthing /bin/syncthing
 
-RUN apk update \
+RUN apk add --no-cache su-exec
-    && apk add su-exec
 
 ENV STNOUPGRADE=1
 ENV PUID=1000