fix: add local channel for docker builds (#945)

The local folder channel was not included in the command, causing newly
built packages on the `bulk` branch to not be usable to their
descendants on the same worker because of strict channel priority.

bioconda_utils/docker_utils.py

@@ -105,7 +105,7 @@
 # The actual building...
 # we explicitly point to the meta.yaml, in order to keep
 # conda-build from building all subdirectories
-conda mambabuild {self.conda_build_args} {self.container_recipe}/meta.yaml 2>&1
+conda mambabuild -c file://{self.container_staging} {self.conda_build_args} {self.container_recipe}/meta.yaml 2>&1
 
 # copy all built packages to the staging area
 cp /opt/conda/conda-bld/*/*.tar.bz2 {self.container_staging}/{arch}