Update 'Dockerfile'

Dockerfile

@@ -2,7 +2,12 @@ FROM alpine:latest
 
 ARG USERNAME=msmtp
 
+COPY start-container.sh /usr/local/bin/start-container
+
 RUN adduser -D $USERNAME \
     && apk add msmtp
+    && chmod +x /usr/local/bin/start-container
 
-USER $USERNAME
+USER $USERNAME
+
+ENTRYPOINT ["start-container"]