Merge pull request #1081 from fluxcd/alpine-3.15

Update Alpine to 3.15

Author: stefanprodan

Dockerfile

@@ -21,7 +21,7 @@ RUN CGO_ENABLED=0 go build \
     -ldflags "-s -w -X github.com/fluxcd/flagger/pkg/version.REVISION=${REVISON}" \
     -a -o flagger ./cmd/flagger
 
-FROM alpine:3.14
+FROM alpine:3.15
 
 RUN apk --no-cache add ca-certificates