Remove nixpkgs version fallback and delete nettest container

Nix builds now require an explicit version = "..." in default.nix —
no implicit nixpkgs eval fallback. Also removes nettest (outgrown)
and updates all doc/task references.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.forgejo/workflows/build-container.yaml

@@ -139,18 +139,12 @@ jobs:
           CONTAINER="${{ matrix.container }}"
           NIX_FILE="containers/$CONTAINER/default.nix"
 
-          # Try extracting version = "..." from the nix file
+          # Extract version = "..." from the nix file
           VERSION=$(grep -m1 '^\s*version\s*=\s*"' "$NIX_FILE" \
             | sed 's/.*"\(.*\)".*/\1/' || true)
 
-          # Fall back to nix eval for nixpkgs packages
           if [ -z "$VERSION" ]; then
-            VERSION=$(nix --extra-experimental-features "nix-command flakes" \
-              eval --raw "nixpkgs#${CONTAINER}.version")
-          fi
-
-          if [ -z "$VERSION" ]; then
-            echo "Error: Could not determine version for $CONTAINER"
+            echo "Error: No version declaration found in $NIX_FILE"
             exit 1
           fi