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>

install.sh

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
-# Containr one-shot installer: creates .env with generated secrets, builds
-# images, boots the stack, and waits for the API to report healthy.
+# Containr one-shot installer: creates .env with generated secrets, pulls the
+# published images (or builds from source), boots the stack, and waits for
+# the API to report healthy.
 set -euo pipefail
 
 cd "$(dirname "$0")"
@@ -45,9 +46,18 @@ fi
 HTTP_PORT="${HTTP_PORT:-3000}"
 API_PORT="${API_PORT:-8082}"
 
-# Build and boot
-info "Building images and starting the stack..."
-docker compose up -d --build
+# Boot: pull published :latest images by default (CONTAINR_VERSION pins a
+# release). Build from source instead with CONTAINR_BUILD=1, or as fallback
+# when the registry is unreachable.
+if [[ "${CONTAINR_BUILD:-}" == "1" ]]; then
+    info "CONTAINR_BUILD=1 - building images from source..."
+    docker compose build
+elif ! docker compose pull; then
+    info "Image pull failed - building from source instead..."
+    docker compose build
+fi
+info "Starting the stack..."
+docker compose up -d
 
 # Wait for the API
 info "Waiting for the API on http://localhost:${API_PORT}/health ..."