fix Dockerfile group name

Author: maorfr

Dockerfile

@@ -1,7 +1,7 @@
 FROM alpine:3.8
 RUN apk --no-cache add ca-certificates
 COPY cain /usr/local/bin/cain
-RUN addgroup -g 1001 -S skbn \
+RUN addgroup -g 1001 -S cain \
     && adduser -u 1001 -D -S -G cain cain
 USER cain
 WORKDIR /home/cain