Pin matrix-media-repo to latest working commit

Current state of the matrix-media-repo master branch is broken.
This change pins the pipeline to the latest working commit.

.drone.yml

@@ -10,7 +10,7 @@ steps:
   image: alpine/git
   commands:
     - git clone https://github.com/turt2live/matrix-media-repo.git .
-    - git checkout master
+    - git checkout 219e4bc
 - name: docker
   image: plugins/docker
   settings:

README.md

@@ -1,3 +1,3 @@
 # docker-image-pipelines
 
-DroneCI pipelines to build Docker images of public projects
+DroneCI pipelines to build Docker images of public projects for my own services