Build using auto-generated Docker image name

This allows running multiple builds of the same pipeline on one Docker
daemon, and also makes configuration simpler, as the  argument is
implied if not provided.

Signed-off-by: Joe Groocock <me@frebib.net>

.drone.yml

@@ -12,13 +12,11 @@ steps:
   image: spritsail/docker-build
   settings:
     make: true
-    repo: plex-dev
 
 - name: test-bin
   pull: always
   image: spritsail/docker-test
   settings:
-    repo: plex-dev
     run: |
       curl --version &&
       xmlstarlet --version
@@ -32,14 +30,12 @@ steps:
     pipe: |
       xmlstarlet sel -t -v "/MediaContainer/@version" |
       grep -qw "$(label io.spritsail.version.plex | cut -d- -f1)"
-    repo: plex-dev
     retry: 10
 
 - name: publish
   pull: always
   image: spritsail/docker-publish
   settings:
-    from: plex-dev
     repo: spritsail/plex-media-server
     tags:
     - latest