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; 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

Dockerfile

@@ -67,13 +67,12 @@ RUN adduser -D -H -s /sbin/nologin pixad && \
     mkdir -p /var/lib/pixa /etc/pixa && \
     chown pixad:pixad /var/lib/pixa
 
-# Copy the image config; signing_key comes from PIXA_SIGNING_KEY.
-# Mount a file over /etc/pixa/config.yml to override anything else.
-COPY config.docker.yml /etc/pixa/config.yml
-
 USER pixad
 WORKDIR /var/lib/pixa
 
 EXPOSE 8080
 
-ENTRYPOINT ["/usr/local/bin/pixad", "--config", "/etc/pixa/config.yml"]
+# Settings come from PORT and the PIXA_ environment variables; only
+# PIXA_SIGNING_KEY is required. A config file mounted at
+# /etc/pixa/config.yml is optional and is read when present.
+ENTRYPOINT ["/usr/local/bin/pixad"]