change Dockerfile to run app under non-root user (#922)

Dockerfile

@@ -1,9 +1,20 @@
 FROM alpine:3.18
 
-RUN apk --no-cache upgrade && apk --no-cache add ca-certificates
+ARG USER=centrifugo
+ARG UID=1000
+ARG GID=1000
 
-COPY centrifugo /usr/local/bin/centrifugo 
+RUN addgroup -S -g $GID $USER && \
+    adduser -S -G $USER -u $UID $USER
+
+RUN apk --no-cache upgrade && \
+    apk --no-cache add ca-certificates && \
+    update-ca-certificates
+
+USER $USER
 
 WORKDIR /centrifugo
 
+COPY centrifugo /usr/local/bin/centrifugo
+
 CMD ["centrifugo"]