The Go implementation of gzip is the only known to produce compressed
layers with the expected digest hashes.
This change allows compressed tar layer files to be produced, which is
useful for exporting layers from non-Go tools.
This fixes a new go-vet(1) error which has surfaced in Go HEAD.
$ go vet ./...
go build github.com/vbatts/tar-split: no non-test Go files in
/home/travis/gopath/src/github.com/vbatts/tar-split
Signed-off-by: Aleksa Sarai <asarai@suse.de>
