Unified dockerfile to latest stable alpine

Dockerfile

@@ -1,9 +1,10 @@
-# Docker image for the Drone build runner
-#
-#     CGO_ENABLED=0 go build -a -tags netgo
-#     docker build --rm=true -t plugins/drone-s3-sync .
+FROM alpine:3.4
+
+RUN apk update && \
+  apk add \
+    ca-certificates \
+    mailcap && \
+  rm -rf /var/cache/apk/*
 
-FROM alpine:3.3
-RUN apk update && apk add ca-certificates mailcap && rm -rf /var/cache/apk/*
 ADD drone-s3-sync /bin/
 ENTRYPOINT ["/bin/drone-s3-sync"]