Keep config.yml out of git and the Docker build context (closes #212)

Getting Started has you create config.yml at the repository root with a
real signing key, but neither .gitignore nor .dockerignore left it out,
so it could be committed and, through COPY . ., reach a build-stage
layer. .gitignore now ignores it next to config.yaml, and .dockerignore
leaves it out in every directory and in any letter case, as it already
does config.yaml and config.dev.yml.

Model: opus-5-5

.gitignore

@@ -37,5 +37,6 @@ node_modules/
 *.sqlite3
 
 # Local dev configs
+config.yml
 config.yaml
 config.dev.yml