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/SETUP.md b/SETUP.md
index 4cb8aa8..4e7dd93 100755
--- a/SETUP.md
+++ b/SETUP.md
@@ -12,7 +12,10 @@ Time units:
 * `uri` — Instance URI. Example: `"uri":"/art/"` -> https://skunky.ebloid.ru/art/
 * `cache` — Caching system; default is off.
   * `enabled` — Caching system state, requires boolean value
-  * `path` — Path to cache directory, requires absolute filesystem path
+  * `path` — Path to cache directory. It must be writable by the user SkunkyArt
+    runs as, and SkunkyArt refuses to start if it is not. The container image
+    runs as uid 10000, so a bind-mounted cache needs
+    `sudo chown -R 10000:10000 <dir>` on the host.
   * `lifetime` — Cached file life time, requires numeric value, followed by multiplicative suffix (see Time Units for details)
   * `max-size` — Maximum file size in megabytes
   * `update-interval` — Automatic rotation interval