Support podman --remote when Containerfile is not in context directory

Fixes: https://github.com/containers/podman/issues/18239

[NO NEW TESTS NEEDED]

@test "podman build -f test" in test/system/070-build.bats

Will test this.  This was passing when run on a local system since
the remote end was using the clients path to read the Containerfile
The issue is it would not work in a podman machine since the
Containerfile would/should be a different path.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

pkg/util/utils.go

@@ -94,6 +94,7 @@ func ParseDockerignore(containerfiles []string, root string) ([]string, string,
 		// so remote must support parsing that.
 		if dockerIgnoreErr != nil {
 			for _, containerfile := range containerfiles {
+				containerfile = strings.TrimPrefix(containerfile, root)
 				if _, err := os.Stat(filepath.Join(root, containerfile+".containerignore")); err == nil {
 					path, symlinkErr = securejoin.SecureJoin(root, containerfile+".containerignore")
 					if symlinkErr == nil {