Bump Alpine 3.12 -> 3.x

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Author: troglobit

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3
 RUN apk add --update git build-base automake autoconf
 
 RUN git clone --depth=1 https://github.com/troglobit/mcjoin.git /root/mcjoin
@@ -8,7 +8,7 @@ RUN ./autogen.sh
 RUN ./configure --prefix=/usr
 RUN make
 
-FROM alpine:3.12
+FROM alpine:3
 COPY --from=0 /root/mcjoin/src/mcjoin /usr/bin/mcjoin
 
 CMD [ "/usr/bin/mcjoin" ]