Daemon: Replace gosu with su-exec.

Pratically the same thing but is available in the regular Alpine repo
and is a bit smaller.

daemon/Dockerfile

 FROM alpine:3.18
 # hadolint ignore=DL3018
-RUN apk add --update --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/testing/ \
-        gosu \
-    && \
-    apk add --update --no-cache \
+RUN apk add --update --no-cache \
         python3 \
+        su-exec \
         transmission-daemon \
     && \
     mkdir -m 777 /run/transmission

daemon/entrypoint

@@ -11,7 +11,7 @@ then
     uid="$(stat ./Downloads -c '%u')"
     gid="$(stat ./Downloads -c '%g')"
     chown "$uid:$gid" ./
-    eval "exec gosu $uid:$gid $*"
+    eval "exec su-exec $uid:$gid $*"
 else
     eval "exec $*"
 fi