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

config.example.yml

@@ -1,4 +1,13 @@
 # Pixa Example Configuration
+#
+# Every key can also be set by an environment variable, which wins over
+# this file: PIXA_ plus the key in upper case, with "." written as "_"
+# (state_dir is PIXA_STATE_DIR, metrics.username is
+# PIXA_METRICS_USERNAME). The one exception is port, which is set by
+# PORT. In a variable, a list is comma-separated. A variable named in
+# this file's env: section is set while the file loads, so it overrides
+# both the environment the process was started with and this file's own
+# key.
 
 # Server settings
 port: 8080