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

.dockerignore

@@ -68,5 +68,6 @@
 /data
 
 # Local config files, kept out of git because they can hold the signing key.
+**/[cC][oO][nN][fF][iI][gG].[yY][mM][lL]
 **/[cC][oO][nN][fF][iI][gG].[yY][aA][mM][lL]
 **/[cC][oO][nN][fF][iI][gG].[dD][eE][vV].[yY][mM][lL]