fix: refuse to start when the cache directory is not writable

An unwritable cache directory degraded silently: media still served, because
the download succeeds before the cache write is attempted, so the only symptom
was one "permission denied" line per request and a cache that never filled.
Every request re-fetched from the CDN.

Probe the directory at startup and exit with the uid and the chown that fixes
it. The container image runs as uid 10000, which is the usual cause with a
bind-mounted cache, so say so in the message and in both compose examples.

1 files changed, 4 insertions, 1 deletions

diff --git a/compose.example.yaml b/compose.example.yaml
index 2e2f088..d7c20ea 100755
--- a/compose.example.yaml
+++ b/compose.example.yaml
@@ -14,4 +14,7 @@ services:
       - no-new-privileges:true
     volumes:
       - ./config.json:/config.json:ro
-      - ./cache:/cache # Ensure cache folder has a 10000:10000 ownership.
+      # The image runs as uid 10000, so the host cache dir must be writable by it:
+      #   mkdir -p cache && sudo chown -R 10000:10000 cache
+      # Without this the container exits at startup telling you the same thing.
+      - ./cache:/cache