Keep docker image up to date

For the sake of security, we should use an updated Alpine image.
Even though we only use "git" from Alpine, Katafygio being
staticaly linked.

Author: bpineau

Dockerfile

@@ -6,7 +6,7 @@ RUN make deps
 RUN make build
 
 FROM alpine:3.7
-RUN apk --no-cache add ca-certificates git
+RUN apk upgrade --no-cache && apk --no-cache add ca-certificates git
 COPY --from=builder /go/src/github.com/bpineau/katafygio/katafygio /usr/bin/
 USER nobody
 ENTRYPOINT ["/usr/bin/katafygio"]

assets/Dockerfile.goreleaser

@@ -1,5 +1,5 @@
 FROM alpine:3.7
-RUN apk --no-cache add ca-certificates git
+RUN apk upgrade --no-cache && apk --no-cache add ca-certificates git
 COPY katafygio /usr/bin/
 USER nobody
 ENTRYPOINT ["/usr/bin/katafygio"]