Use pre-built mdbook image for manual deployment

.woodpecker/build.yml

@@ -51,15 +51,13 @@ pipeline:
       event: [push, pull_request]
 
   deployManual:
-    image: rust:1.54.0-slim
+    image: docker.bics.ga/rei_ci/mdbook
     when:
       branch:
         - develop
     secrets:
       - deploy_ssh_key
     commands: # warning! Doesn't seem to like hashed known_hosts...
-      - cargo install -q mdbook
-      - DEBIAN_FRONTEND=noninteractive apt-get -qq update && apt-get -yqq install rsync ssh > /dev/null
       - mdbook build
       - echo "$DEPLOY_SSH_KEY" > /tmp/rsync_key
       - chmod u=rw,go= /tmp/rsync_key