Keep local config files out of the Docker build context (closes #211)

config.yaml and config.dev.yml are kept out of git because they can hold
the signing key, but .dockerignore did not leave them out, so a local
copy in the working tree reached the build context and, through
COPY . ., a build-stage layer. .dockerignore now leaves them out in
every directory and in any letter case. configs/config.example.yml is
still sent.

Model: opus-5-5

.dockerignore

@@ -66,3 +66,7 @@
 .gitignore
 /bin
 /data
+
+# Local config files, kept out of git because they can hold the signing key.
+**/[cC][oO][nN][fF][iI][gG].[yY][aA][mM][lL]
+**/[cC][oO][nN][fF][iI][gG].[dD][eE][vV].[yY][mM][lL]