build: add postal ./bin to path and set config root in container

.dockerignore

@@ -1,22 +1,19 @@
-/.bundle
 .byebug_history
-config/postal.yml
-config/smtp.cert
-config/smtp.key
-config/signing.key
-public/assets
-vendor/bundle
-Procfile.local
-log/*
-storage/*
-tmp/*
+.cache
 .git/*
 .github/*
 .vscode/*
 .yardoc/*
+/.bundle
+config/postal/*
 doc/*
-Procfile*
-node_modules
 Dockerfile
+log/*
+node_modules
+Procfile.local
+Procfile*
+public/assets
+storage/*
+tmp/*
+vendor/bundle
 vendor/bundle
-.cache