Enable DNS resolution in "make shell"

Networking is currently enabled in test containers (we don't use
--unshare-net) but for domain names to be resolvable, we need an
/etc/resolv.conf, too, so add it from the host, similar to how e.g.
Podman does it.

We don't need network access in the test-suite but it can be handy for
interactive testing in "make shell", e.g. to allow gdb to fetch
debuginfo data through debuginfod automatically.

tests/mktree.fedora

@@ -148,6 +148,9 @@ case $CMD in
             zlib \
             zstd
 
+        # Enable DNS resolution
+        cp /etc/resolv.conf $BASE_DIR/etc/
+
         # Point RPM to newly created database
         echo "%_dbpath $(rpm --eval '%_dbpath')" > $BASE_DIR/root/.rpmmacros