replace some basic uses of fmt.Sprintf()

Really tiny gains here, and doesn't significantly impact readability:

    BenchmarkSprintf
    BenchmarkSprintf-10    11528700     91.59 ns/op   32 B/op  1 allocs/op
    BenchmarkConcat
    BenchmarkConcat-10    100000000     11.76 ns/op    0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

remotes/docker/handler.go

@@ -103,7 +103,7 @@ func appendDistributionSourceLabel(originLabel, repo string) string {
 }
 
 func distributionSourceLabelKey(source string) string {
-	return fmt.Sprintf("%s.%s", labels.LabelDistributionSource, source)
+	return labels.LabelDistributionSource + "." + source
 }
 
 // selectRepositoryMountCandidate will select the repo which has longest