Use the host's transmission user.

daemon/Dockerfile

 FROM alpine:3.13
-# hadolint ignore=DL3018
+# hadolint ignore=DL3018,SC2174
 RUN apk add --update --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/testing/ \
         gosu \
     && \
     apk add --update --no-cache \
         transmission-daemon \
-        shadow \
     && \
-    groupmod -g 1000 transmission && \
-    usermod -u 1000 transmission && \
-    apk del --no-cache --rdepends shadow && \
-    mkdir -p /var/run/transmission /etc/transmission/ && \
-    chown -R transmission:transmission /var/run/transmission
+    mkdir -pm 777 /run/transmission && \
+    mkdir -pm 777 /var/lib/transmission && \
+    mkdir -p /etc/transmission
 COPY --chown=root:root entrypoint /
-COPY --chown=1000:1000 settings.json /etc/transmission/
+COPY settings.json /etc/transmission/
 VOLUME /var/lib/transmission/
 WORKDIR /var/lib/transmission
 EXPOSE 9091 51413/tcp 51413/udp

docker-compose.yml

@@ -9,6 +9,8 @@ services:
       - 51413:51413/tcp
       - 51413:51413/udp
     volumes:
+      - /etc/group:/etc/group:ro
+      - /etc/passwd:/etc/passwd:ro
       - daemon:/var/lib/transmission
       - /srv/library:/var/lib/transmission/downloads
     restart: always