docs: add app screenshots, default compose to published :latest images

- docker-compose.yml now references ghcr.io/dvorinka/containr-*:latest with
  pull_policy: missing, so `docker compose up` pulls published images by
  default; CONTAINR_VERSION pins a release tag, build config retained for
  source builds
- install.sh pulls instead of always building (CONTAINR_BUILD=1 or a failed
  pull falls back to a source build)
- README gains a Screenshots section (dashboard, canvas, service detail,
  databases) captured from a live local stack, plus updated install docs

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.env.example

@@ -11,6 +11,10 @@ API_PORT=8082
 POSTGRES_PORT=5432
 REDIS_PORT=6379
 
+# Image tag to pull from GHCR (e.g. latest, 0.1.1). Use CONTAINR_BUILD=1
+# with install.sh to build from source instead.
+CONTAINR_VERSION=latest
+
 # Database Configuration
 POSTGRES_DB=containr
 POSTGRES_USER=containr_user