fix mod flag in docker build after #87 changes

Author: ngraef

Dockerfile

@@ -16,7 +16,7 @@ WORKDIR /go/src/sigs.k8s.io/aws-encryption-provider
 ARG TAG
 COPY . ./
 ENV GO111MODULE=on
-RUN	CGO_ENABLED=0 GOOS=linux go build -mod vendor -ldflags \
+RUN	CGO_ENABLED=0 GOOS=linux go build -mod mod -ldflags \
     "-w -s -X sigs.k8s.io/aws-encryption-provider/pkg/version.Version=$TAG" \
     -o bin/aws-encryption-provider cmd/server/main.go