Check schema correctness also with libxml2/xmllint

Why both? The stars show their strong points:

|                            | jing      | xmllint         |
|----------------------------+-----------+-----------------|
| runtime dependencies       | big (JRE) | * small (glibc) |
| detects invalid documents  | yes       | yes             |
| explains what is wrong     |           |                 |
| ... with invalid documents | * yes     | no              |

Dockerfile

@@ -1,6 +1,7 @@
 FROM registry.opensuse.org/yast/head/containers/yast-ruby:latest
 RUN zypper --gpg-auto-import-keys --non-interactive in --no-recommends \
   autoyast2 \
+  libxml2-tools \
   jing \
   trang \
   yast2 \

package/yast2-schema.changes

@@ -2,6 +2,7 @@
 Thu May  7 16:13:37 UTC 2020 - Imobach Gonzalez Sosa <igonzalezsosa@suse.com>
 
 - Allow optional types for string and map objects (bsc#1170886).
+- Check schema correctness with jing and xmllint (bsc#1172131).
 - 4.3.0
 
 -------------------------------------------------------------------

package/yast2-schema.spec

@@ -30,6 +30,7 @@ Url:            https://github.com/yast/yast-schema
 
 # Dependencies needed to build the package
 BuildRequires:  jing
+BuildRequires:  libxml2-tools
 BuildRequires:	trang yast2-devtools
 
 # All packages providing RNG files for AutoYaST

src/rng/Makefile.am

@@ -19,6 +19,7 @@ clean-local:
 # Test with a minimal valid profile that the schema is correct
 check-local:
 	echo '<profile xmlns="http://www.suse.com/1.0/yast2ns"/>' | jing profile.rng /dev/stdin
+	echo '<profile xmlns="http://www.suse.com/1.0/yast2ns"/>' | xmllint --noout --relaxng profile.rng /dev/stdin
 
 install-data-local:
 	$(INSTALL) -d $(DESTDIR)$(schemadir)/autoyast/rng