Every setting can be given as an environment variable (closes #128)

Each config key can now be set by PIXA_ plus the key in upper case
("." written as "_"), and the port by PORT. A present variable, even
an empty one, is read before the config file through the existing
typed getters, so every existing check covers it; errors name the key
and the variable, never the signing key or metrics password. A
variable named in the file's env: section overrides both. An empty
string for blocked_networks or trusted_proxies is now an empty list.
The image no longer bakes in config.docker.yml or passes --config; its
HEALTHCHECK probes ${PORT:-8080}. The config file is looked for under
/etc/pixa rather than /etc/pixad. Also covers #99.

Model: opus-5-5

TODO.md

@@ -30,6 +30,18 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 every setting as an environment variable (closes #128, also
+  covers #99): each config key can be set by `PIXA_` plus the key in upper
+  case (`.` written as `_`), and the port by `PORT`; a variable present in
+  the environment, even empty, wins over the config file, which wins over
+  the default; the typed getters read the variable first, so every existing
+  check applies to it and a bad value aborts startup naming the variable;
+  lists are comma-separated, and an empty variable (or `""` in the file) is
+  an empty list; the Docker image no longer bakes in `config.docker.yml` or
+  passes `--config`, and its `HEALTHCHECK` probes `PORT` (default `8080`);
+  the config file is looked for under `/etc/pixa` and `~/.config/pixa`
+  instead of the daemon name `pixad`; documented in `README.md` and
+  `config.example.yml`.
 - 2026-09-28 quality and fit in the URL signature (closes #60): the signed
   data is now `host:path:query:width:height:format:expiration:quality:fit`,
   using `85` and `cover` when the URL has no `q` or `fit`, so one signed
@@ -178,7 +190,6 @@ exhaustion
 - P2: auto format selection (format=auto based on Accept header)
 - P2: configuration
     - add all configuration options from README
-    - environment variable overrides
     - YAML config file support
 - P2: operational
     - optional Sentry error reporting