diff --git a/README.md b/README.md
index f37a5bd..ec6f426 100644
--- a/README.md
+++ b/README.md
@@ -55,18 +55,21 @@ chroot_exec rc-update add example_daemon default
 The following variables can be used to modify the base behaviour of the image 
 builder.
 
-| Variable              | Default Value                        | Description                      |
-| --------------------- | ------------------------------------ | -------------------------------- |
-| ALPINE_MIRROR         | http://dl-cdn.alpinelinux.org/alpine | Mirror used for package download |
-| ALPINE_BRANCH         | 3.10                                 | [Alpine Branch](https://wiki.alpinelinux.org/wiki/Alpine_Linux:Releases) to use for image |
+| Variable              | Default Value                        | Description                                                                                       |
+| --------------------- | ------------------------------------ | ------------------------------------------------------------------------------------------------- |
+| ALPINE_BRANCH         | 3.10                                 | [Alpine Branch](https://wiki.alpinelinux.org/wiki/Alpine_Linux:Releases) to use for image         |
+| ALPINE_MIRROR         | http://dl-cdn.alpinelinux.org/alpine | Mirror used for package download                                                                  |
+| CUSTOM_IMAGE_SCRIPT   | image.sh                             | Name of script for image customizations (relative to input dir)                                   |
+| DEFAULT_HOSTNAME      | alpine                               | Default hostname                                                                                  |
+| DEFAULT_ROOT_PASSWORD | alpine                               | Default password for root user                                                                    |
 | DEFAULT_TIMEZONE      | Etc/UTC                              | Default [Timezone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) to use for image |
-| DEFAULT_HOSTNAME      | alpine                               | Default hostname |
-| DEFAULT_ROOT_PASSWORD | alpine                               | Default password for root user |
-| SIZE_BOOT             | 100M                                 | Size of boot partition |
-| SIZE_ROOT_FS          | 200M                                 | Size of root file system |
-| SIZE_ROOT_PART        | 500M                                 | Size of root partition |
-| SIZE_DATA             | 20M                                  | Initial Size of data partition |
-| IMG_NAME              | sdcard                               | Base name of created image file |
+| IMG_NAME              | sdcard                               | Base name of created image file                                                                   |
+| INPUT_PATH            | /input                               | Input directory inside container                                                                  |
+| OUTPUT_PATH           | /output                              | Output directory inside container                                                                 |
+| SIZE_BOOT             | 100M                                 | Size of boot partition                                                                            |
+| SIZE_DATA             | 20M                                  | Initial Size of data partition                                                                    |
+| SIZE_ROOT_FS          | 200M                                 | Size of root file system                                                                          |
+| SIZE_ROOT_PART        | 500M                                 | Size of root partition                                                                            |
 
 ### Update running system
 
diff --git a/resources/build.sh b/resources/build.sh
index 45b4a12..9a1ed67 100755
--- a/resources/build.sh
+++ b/resources/build.sh
@@ -17,6 +17,10 @@ set -e
 : ${SIZE_DATA:="20M"}
 : ${IMG_NAME:="sdcard"}
 
+: ${OUTPUT_PATH:="/output"}
+: ${INPUT_PATH:="/input"}
+: ${CUSTOM_IMAGE_SCRIPT:="image.sh"}
+
 
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
 # static config
@@ -25,8 +29,6 @@ RES_PATH=/resources/
 BASE_PACKAGES="alpine-base tzdata parted ifupdown e2fsprogs-extra util-linux coreutils linux-rpi linux-rpi2"
 
 WORK_PATH="/work"
-OUTPUT_PATH="/output"
-INPUT_PATH="/input"
 ROOTFS_PATH="${WORK_PATH}/root_fs"
 BOOTFS_PATH="${WORK_PATH}/boot_fs"
 DATAFS_PATH="${WORK_PATH}/data_fs"
@@ -327,8 +329,8 @@ mkdir -p ${DATAFS_PATH}
 # Custom modification
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
 
-if [ -f ${INPUT_PATH}/image.sh ]; then
-  . ${INPUT_PATH}/image.sh
+if [ -f ${INPUT_PATH}/${CUSTOM_IMAGE_SCRIPT} ]; then
+  . ${INPUT_PATH}/${CUSTOM_IMAGE_SCRIPT}
 fi
 
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #