feat: ship image config that reads signing_key from the environment

The runtime stage now copies config.docker.yml, which sets only
signing_key (from PIXA_SIGNING_KEY), state_dir, and port. This drops
the public placeholder key and the baked-in allowlist from the image,
matching how upaas configures apps: environment variables and mounts,
no injected config file.

Model: opus-4-8

config.docker.yml

@@ -0,0 +1,11 @@
+# Pixa configuration baked into the Docker image.
+#
+# The signing key is read from the PIXA_SIGNING_KEY environment
+# variable; startup aborts naming it when it is unset. Every other key
+# is omitted so its default applies. Operators who need more (an
+# allowlist, metrics, and so on) mount their own file over
+# /etc/pixa/config.yml.
+
+signing_key: "${ENV:PIXA_SIGNING_KEY}"
+state_dir: /var/lib/pixa
+port: 8080