Run make devserver from environment variables, not a config file (closes #360)

The target mounted config.dev.yml, which nothing creates, so Docker made
a directory in its place and pixad exited. It now passes a new random
PIXA_SIGNING_KEY each run, printed so the developer can log in, and
PIXA_DEBUG=true so the login form works over plain HTTP. The unused
.gitignore entry for config.dev.yml is removed.

Model: opus-5-5

.gitignore

@@ -70,4 +70,3 @@ vendor/
 # Local dev configs
 config.yml
 config.yaml
-config.dev.yml