diff --git a/README.md b/README.md
index 1d5217c..f3671c1 100644
--- a/README.md
+++ b/README.md
@@ -30,6 +30,27 @@ $ pack build --buildpack fagiani/apt myapp
     # or add custom apt repos (only required if using packages outside of the standard Ubuntu APT repositories)
     :repo:deb http://cz.archive.ubuntu.com/ubuntu artful main universe
 
+    # for repos without a GPG key in the builder, use [trusted=yes] to skip signature verification
+    :repo:deb [trusted=yes] http://ppa.launchpad.net/deadsnakes/ppa/ubuntu jammy main
+
+## Custom Repository Order
+
+By default, custom repositories defined with `:repo:` in your `Aptfile` are **appended** after the system's default repositories. You can control this behavior using the `APT_CUSTOM_REPOS_MODE` environment variable:
+
+| Value | Behavior |
+|-------|----------|
+| `append` (default) | System repos first, then custom repos |
+| `prepend` | Custom repos first, then system repos |
+| `custom-only` | Only custom repos are used (system repos are excluded entirely) |
+
+Example:
+
+```
+$ pack build --buildpack fagiani/apt --env APT_CUSTOM_REPOS_MODE=prepend myapp
+```
+
+> **Note:** On Ubuntu 24.04-based builders (e.g., `heroku/builder:24`), system repositories use the deb822 format in `/etc/apt/sources.list.d/`. In `append` and `prepend` modes, custom repos are placed in `sources.list` alongside the system's `sources.list.d/` entries. In `custom-only` mode, both `sources.list` and `sources.list.d/` are replaced with only your custom repositories.
+
 ## License
 
 MIT
diff --git a/bin/build b/bin/build
index 02db9af..18f67ac 100755
--- a/bin/build
+++ b/bin/build
@@ -73,15 +73,58 @@ else
   mkdir -p "$APT_SOURCELIST_DIR"   # make dir for sources
   mkdir -p "$APT_SOURCEPARTS_DIR" # make dir for source parts
   cp -f Aptfile "$apt_layer"/Aptfile
-  cat "/etc/apt/sources.list" > "$APT_SOURCES"    # no cp here
-  if compgen -G "/etc/apt/sources.list.d/*" > /dev/null; then
-    cp /etc/apt/sources.list.d/* "$APT_SOURCEPARTS_DIR"/
-  fi
+
   # add custom repositories from Aptfile to sources.list
   # like>>    :repo:deb http://cz.archive.ubuntu.com/ubuntu artful main universe
   if grep -q -e "^:repo:" Aptfile; then
     topic "Adding custom repositories"
-    cat Aptfile | grep -s -e "^:repo:" | sed 's/^:repo:\(.*\)\s*$/\1/g' >> $APT_SOURCES
+
+    # Capture custom repositories
+    CUSTOM_REPOS=$(grep -s -e "^:repo:" Aptfile | sed 's/^:repo:\(.*\)\s*$/\1/g')
+
+    # Check if system sources.list has actual repo lines (not just comments)
+    SYSTEM_SOURCES_ACTIVE=$(grep -v -e '^\s*#' -e '^\s*$' /etc/apt/sources.list 2>/dev/null || true)
+
+    # Determine repository order
+    case "${APT_CUSTOM_REPOS_MODE:-append}" in
+      "prepend")
+        if compgen -G "/etc/apt/sources.list.d/*" > /dev/null; then
+          cp /etc/apt/sources.list.d/* "$APT_SOURCEPARTS_DIR"/
+        fi
+        if [ -n "$SYSTEM_SOURCES_ACTIVE" ]; then
+          # Traditional format (builder:22) — system repos in sources.list
+          { echo "$CUSTOM_REPOS"; echo ""; echo "$SYSTEM_SOURCES_ACTIVE"; } > "$APT_SOURCES"
+        else
+          # deb822 format (builder:24) — system repos in sources.list.d/
+          # Put custom repos in sources.list (apt reads it before sources.list.d/)
+          echo "$CUSTOM_REPOS" > "$APT_SOURCES"
+        fi
+        ;;
+      "custom-only")
+        echo "$CUSTOM_REPOS" > "$APT_SOURCES"
+        # Do not copy sources.list.d — only custom repos should be used
+        ;;
+      *)  # Default to append
+        if compgen -G "/etc/apt/sources.list.d/*" > /dev/null; then
+          cp /etc/apt/sources.list.d/* "$APT_SOURCEPARTS_DIR"/
+        fi
+        if [ -n "$SYSTEM_SOURCES_ACTIVE" ]; then
+          # Traditional format (builder:22) — system repos in sources.list
+          { echo "$SYSTEM_SOURCES_ACTIVE"; echo ""; echo "$CUSTOM_REPOS"; } > "$APT_SOURCES"
+        else
+          # deb822 format (builder:24) — system repos in sources.list.d/
+          # Put custom repos in a file that sorts after system sources (zzz-custom.list)
+          : > "$APT_SOURCES"  # empty sources.list
+          echo "$CUSTOM_REPOS" > "$APT_SOURCEPARTS_DIR/zzz-custom.list"
+        fi
+        ;;
+    esac
+  else
+    # No custom repos — use system defaults as-is
+    cat "/etc/apt/sources.list" > "$APT_SOURCES"
+    if compgen -G "/etc/apt/sources.list.d/*" > /dev/null; then
+      cp /etc/apt/sources.list.d/* "$APT_SOURCEPARTS_DIR"/
+    fi
   fi
 
   APT_OPTIONS="-o debug::nolocking=true -o dir::cache=$APT_CACHE_DIR -o dir::state=$APT_STATE_DIR"