osbuild: Move /var in BuildRoot outside of tmpfs

In BuildRoot a new mount /var pointing to temporary directory in host's
/var/tmp is created. This enables us to have temporary storage inside
the container which is not hosted on tmpfs. Thanks to that we can move
larger files out of the part of filesystem which is hosted on tmpfs to
save up memory on machines with low memory capacity.

osbuild/buildroot.py

@@ -16,7 +16,13 @@ class BuildRoot:
     def __init__(self, root, path="/run/osbuild"):
         self.root = tempfile.mkdtemp(prefix="osbuild-buildroot-", dir=path)
         self.api = tempfile.mkdtemp(prefix="osbuild-api-", dir=path)
+        self.var = tempfile.mkdtemp(prefix="osbuild-var-", dir="/var/tmp")
         self.mounts = []
+
+        self.mount_root(root)
+        self.mount_var()
+
+    def mount_root(self, root):
         for p in ["usr", "bin", "sbin", "lib", "lib64"]:
             source = os.path.join(root, p)
             target = os.path.join(self.root, p)
@@ -30,6 +36,16 @@ class BuildRoot:
                 raise
             self.mounts.append(target)
 
+    def mount_var(self):
+        target = os.path.join(self.root, "var")
+        os.mkdir(target)
+        try:
+            subprocess.run(["mount", "-o", "bind", self.var, target], check=True)
+        except subprocess.CalledProcessError:
+            self.unmount()
+            raise
+        self.mounts.append(target)
+
     def unmount(self):
         for path in self.mounts:
             subprocess.run(["umount", "--lazy", path], check=True)
@@ -41,6 +57,9 @@ class BuildRoot:
         if self.api:
             shutil.rmtree(self.api)
             self.api = None
+        if self.var:
+            shutil.rmtree(self.var)
+            self.var = None
 
     def run(self, argv, binds=None, readonly_binds=None, **kwargs):
         """Runs a command in the buildroot.