Use host system OSTree repo as local cache

When running the image builder on Endless OS, there is a high chance
that the host system will be running a similar, if not identical, OSTree
commit to the one being built into the image. In my case, I run master
and update essentially every morning, so running the image builder with
its default arguments will use the same OSTree as is booted.

`ostree pull` supports a `--localcache-repo PATH` argument:

> Like git's clone --reference. Reuse the provided OSTree repo as a
> local object cache when doing HTTP fetches.

In this patch, we bind-mount the host's /ostree/repo into the buildroot,
if it exists. The bind-mount is made read-only to reduce the risk that a
bug in the image builder will mess up the host system.

run-build

@@ -77,6 +77,7 @@ class ImageBuildRoot(object):
         # Optional mounts dependent on existence on the host
         optional_mounts = [
             config['build']['sysconfdir'],
+            '/ostree/repo',
         ]
 
         # Include local settings directory if provided
@@ -179,9 +180,9 @@ class ImageBuildRoot(object):
 
         # Really mount the directories
         for path in to_mount:
-            self.mount(path)
+            self.mount(path, read_only=(path == '/ostree/repo'))
 
-    def mount(self, path, target=None):
+    def mount(self, path, target=None, read_only=False):
         """Bind mount path in the build directory"""
         if not os.path.isabs(path):
             raise eib.ImageBuildError('Buildroot mount path', path,
@@ -206,6 +207,8 @@ class ImageBuildRoot(object):
             with open(mount_path, 'w'):
                 pass
         subprocess.check_call(['mount', '--bind', path, mount_path])
+        if read_only:
+            subprocess.check_call(['mount', '-o', 'remount,bind,ro', path, mount_path])
 
     def __enter__(self):
         return self

stages/eib_ostree

@@ -35,7 +35,14 @@ ostree --repo="${EIB_OSTREE_REPODIR}" remote delete --if-exists \
 ostree --repo="${EIB_OSTREE_REPODIR}" remote add \
   --set=gpg-verify-summary="${gpg_verify_summary}" \
   ${EIB_OSTREE_REMOTE} ${BUILD_OSTREE_URL} ${EIB_OSTREE_REF}
+if [ -d /ostree/repo ]
+then
+  extra_pull_args=(--localcache-repo /ostree/repo)
+else
+  extra_pull_args=()
+fi
 eib_retry ostree --repo="${EIB_OSTREE_REPODIR}" pull \
+  ${extra_pull_args[@]} \
   ${EIB_OSTREE_REMOTE} ${EIB_OSTREE_REF}
 
 # Recreate the ref locally so that the deploy can pull from it