Upgrade docker/distribution to v2.8.1

The 2.8.0 release had an issue with go.sum when using GOPROXY=direct.
The only change in 2.8.1 is addressing this issue.

See here for more details:
distribution/distribution#3596

go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/blang/semver v3.5.1+incompatible
 	github.com/coredns/corefile-migration v1.0.14
 	github.com/davecgh/go-spew v1.1.1
-	github.com/docker/distribution v2.8.0+incompatible
+	github.com/docker/distribution v2.8.1+incompatible
 	github.com/drone/envsubst/v2 v2.0.0-20210730161058-179042472c46
 	github.com/evanphx/json-patch/v5 v5.6.0
 	github.com/fatih/color v1.13.0

go.sum

@@ -180,6 +180,8 @@ github.com/dgryski/go-sip13 v0.0.0-20181026042036-e10d5fee7954/go.mod h1:vAd38F8
 github.com/docker/distribution v2.7.1+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
 github.com/docker/distribution v2.8.0+incompatible h1:l9EaZDICImO1ngI+uTifW+ZYvvz7fKISBAKpg+MbWbY=
 github.com/docker/distribution v2.8.0+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
+github.com/docker/distribution v2.8.1+incompatible h1:Q50tZOPR6T/hjNsyc9g8/syEs6bk8XXApsHjKukMl68=
+github.com/docker/distribution v2.8.1+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
 github.com/docopt/docopt-go v0.0.0-20180111231733-ee0de3bc6815/go.mod h1:WwZ+bS3ebgob9U8Nd0kOddGdZWjyMGR8Wziv+TBNwSE=
 github.com/drone/envsubst/v2 v2.0.0-20210730161058-179042472c46 h1:7QPwrLT79GlD5sizHf27aoY2RTvw62mO6x7mxkScNk0=
 github.com/drone/envsubst/v2 v2.0.0-20210730161058-179042472c46/go.mod h1:esf2rsHFNlZlxsqsZDojNBcnNs5REqIvRrWRHqX0vEU=