address `SQLITE_IOERR_GETTEMPPATH` from docker

guide/install.md

@@ -58,6 +58,12 @@ services:
       # Docker will "helpfully" create a directory by this name otherwise.
       - "/etc/moonfire-nvr.toml:/etc/moonfire-nvr.toml:ro"
 
+      # Pass through `/var/tmp` from the host.
+      # SQLite expects to be able to create temporary files in this dir, which
+      # is not created in Moonfire's minimal Docker image.
+      # See: <https://www.sqlite.org/tempfiles.html>
+      - "/var/tmp:/var/tmp"
+
       # Add additional mount lines here for each sample file directory
       # outside of /var/lib/moonfire-nvr, e.g.:
       # - "/media/nvr:/media/nvr"

guide/troubleshooting.md

@@ -228,6 +228,22 @@ clock_gettime failed: EPERM: Operation not permitted
 This indicates a broken environment. See the troubleshooting guide.
 ```
 
+#### `VFS is unable to determine a suitable directory for temporary files`
+
+Moonfire NVR's database internally uses SQLite, which creates
+[various temporary files](https://www.sqlite.org/tempfiles.html). If it can't
+find a path that exists and is writable by the current user, it will produce
+errors such as the following:
+
+```
+2023-12-29T16:16:47.795330  WARN           sync-1 syncer{path=/media/nvr/sample}: moonfire_db::writer: flush failure on save for reason 120 sec after start of 59 seconds driveway-sub recording 10/1222348; will retry after PT60S: UNAVAILABLE
+caused by: disk I/O error
+caused by: Error code 6410: VFS is unable to determine a suitable directory for temporary files
+```
+
+The simplest solution is to pass `/var/tmp` through from the host to the Docker
+container in your Docker compose file.
+
 ### Server errors
 
 #### `Error: pts not monotonically increasing; got 26615520 then 26539470`