use consistent alias for docker image spec

This package was aliased as "imagespec" in some places, and "dockerspec"
in other places, which made it easy to confuse.

Change all uses of this package to be aliased as "dockerspec" and configure
an "importas" linting check to enforce it.

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

.golangci.yml

@@ -194,6 +194,8 @@ linters:
           alias: c8dimages
         - pkg: github.com/opencontainers/image-spec/specs-go/v1
           alias: ocispec
+        - pkg: github.com/moby/docker-image-spec/specs-go/v1
+          alias: dockerspec
         - pkg: go.etcd.io/bbolt
           alias: bolt
           # Enforce that gotest.tools/v3/assert/cmp is always aliased as "is"