Take every setting from PIXA_ variables and PORT (closes #128)

Each config key can now be set by PIXA_ plus the key in upper case
("." written as "_"), and the port by PORT. One list pairs keys with
variables; the typed getters read a present variable, even an empty
one, before the config file, so every existing check covers it, and
errors a variable can reach name both the key and the variable. An
empty string for blocked_networks or trusted_proxies is now an empty
list, as for allowlist_hosts. The image no longer bakes in
config.docker.yml or passes --config, and its HEALTHCHECK probes
${PORT:-8080}; the config file is looked for under /etc/pixa rather
than /etc/pixad, so a file mounted at /etc/pixa/config.yml is still
read.

Model: opus-5-5

config.example.yml

@@ -1,4 +1,10 @@
 # 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.
 
 # Server settings
 port: 8080