pin to v0.6.4

we needed 'master' in order to use type=oci with named oci outputs, but
now that we're using type=docker we don't need to.

fixes #19

Signed-off-by: Alex Suraci <[email protected]>

Author: vito

Dockerfile

@@ -10,7 +10,7 @@ FROM concourse/golang-builder AS builder
   RUN go build -o /assets/task ./cmd/task
   RUN go build -o /assets/build ./cmd/build
 
-FROM moby/buildkit:master AS task
+FROM moby/buildkit:v0.6.4 AS task
   COPY --from=builder /assets/task /usr/bin/
   COPY --from=builder /assets/build /usr/bin/
   COPY bin/setup-cgroups /usr/bin/