fix: Dockerfile to reduce vulnerabilities

Author: snyk-bot

Dockerfile

@@ -3,6 +3,6 @@ WORKDIR /app
 COPY . .
 RUN go build -o gitoops ./cmd/
 
-FROM alpine:3.14
+FROM alpine:3.15
 COPY --from=0 /app/gitoops /usr/local/bin/
 ENTRYPOINT ["/usr/local/bin/gitoops"]