feat: use slim image (#61)

- mount docker.sock and /tmp to renovate
- use slim image

Co-authored-by: Jeroen de Bruijn <vidavidorra+jdbruijn@gmail.com>
Co-authored-by: Jeroen de Bruijn <62570005+jdbruijn@users.noreply.github.com>

src/docker.ts

@@ -1,7 +1,7 @@
 class Docker {
   readonly repository = 'renovate/renovate';
   // renovate: datasource=docker depName=renovate/renovate versioning=docker
-  readonly tag = '19.231.12';
+  readonly tag = '19.231.12-slim';
 
   image(): string {
     return `${this.repository}:${this.tag}`;