optimize ci build: use own dockerfile for dependencies + cache for builds (#164)

Reviewed-on: Ruderverein-Donau-Linz/rowt#164

README.md

@@ -1,17 +1,14 @@
 # Build
-
 ## Frontend
 1. `cd frontend`
 2. `npm install`
 3. `npm run (watch/build)`
 
 # Run
-
 ## Backend
 1. `cargo r`
 
 # Test
-
 ## Frontend
 - `npx playwright test --workers 1 --project firefox`
 - Nice UI: `--ui`
@@ -19,4 +16,3 @@
 
 ## Backend (Unit + Integration)
 `cargo t`
-