add BUILDER_CONFIG documentation

doc/Configuration.md

@@ -22,12 +22,13 @@
 
 ## `.buildrc` variables
 
-  Builder configuriation is automatically loaded from `$HOME/.buildrc`. If this
-  file does not exist, it will be created and the build will abort for you to
-  check the settings. The variables expected to be available after sourcing
-  `.buildrc` are described in the following table. The variables marked
-  "optional", if undefined, will be filled with appropriate default values by
-  builder for each build.
+  Builder configuriation is automatically loaded from `$HOME/.buildrc` (or the
+  file pointed to by the `$BUILDER_CONFIG` variable). If this file does not
+  exist, it will be created and the build will abort for you to check the
+  settings. The variables expected to be available after sourcing `.buildrc`
+  are described in the following table. The variables marked "optional", if
+  undefined, will be filled with appropriate default values by builder for each
+  build.
 
   Note that the expressions need not be constants, but are evaluated according
   to normal bash expansion rules (see man bash(1)). By this, you can

doc/Install.md

@@ -71,3 +71,20 @@ still available.
 
 After this you can install or load Builder and go on extending your deployment.
 
+
+### Multi-configuration Environments
+
+When using Builder in a multi-user or CI setup it is inconvenient to have the
+`.buildrc` loaded from the user-account home directory. To circumvent this the
+variable `$BUILDER_CONFIG` can be defined to override the default path. For
+example, suppose you keep your configuration in a repository
+`repo/builder.conf`. You can use any Builder installation available and point
+it to your current setup:
+
+```bash
+#cd repo   # wherever this is
+export BUILDER_CONFIG="${PWD}/builder.conf"
+```
+
+With this, `build` can be called from any location and will find the right
+config file.