Update docker distribution to fix go sum error

This change is needed as building fails. According to https://github.com/distribution/distribution/releases/tag/v2.8.1,
there are no changes between 2.8.0 and 2.8.1, the sole reason of having
this new version is to fix the go.sum issue.

go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/blang/semver v3.5.1+incompatible
 	github.com/coredns/corefile-migration v1.0.16
 	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
@@ -159,4 +159,7 @@ require (
 	sigs.k8s.io/structured-merge-diff/v4 v4.2.1 // indirect
 )
 
-require github.com/emicklei/go-restful v2.15.0+incompatible // indirect
+require (
+	github.com/emicklei/go-restful v2.15.0+incompatible // indirect
+	github.com/opencontainers/image-spec v1.0.2 // indirect
+)

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=
@@ -553,6 +555,8 @@ github.com/onsi/gomega v1.17.0 h1:9Luw4uT5HTjHTN8+aNcSThgH1vdXnmdJ8xIfZ4wyTRE=
 github.com/onsi/gomega v1.17.0/go.mod h1:HnhC7FXeEQY45zxNK3PPoIUhzk/80Xly9PcubAlGdZY=
 github.com/opencontainers/go-digest v1.0.0 h1:apOUWs51W5PlhuyGyz9FCeeBIOUDA/6nW8Oi/yOhh5U=
 github.com/opencontainers/go-digest v1.0.0/go.mod h1:0JzlMkj0TRzQZfJkVvzbP0HBR3IKzErnv2BNG4W4MAM=
+github.com/opencontainers/image-spec v1.0.2 h1:9yCKha/T5XdGtO0q9Q9a6T5NUCsTn/DrBg0D7ufOcFM=
+github.com/opencontainers/image-spec v1.0.2/go.mod h1:BtxoFyWECRxE4U/7sNtV5W15zMzWCbyJoFRP3s7yZA0=
 github.com/opentracing/opentracing-go v1.1.0/go.mod h1:UkNAQd3GIcIGf0SeVgPpRdFStlNbqXla1AfSYxPUl2o=
 github.com/pascaldekloe/goe v0.0.0-20180627143212-57f6aae5913c/go.mod h1:lzWF7FIEvWOWxwDKqyGYQf6ZUaNfKdP144TG7ZOy1lc=
 github.com/pascaldekloe/goe v0.1.0/go.mod h1:lzWF7FIEvWOWxwDKqyGYQf6ZUaNfKdP144TG7ZOy1lc=