Updates to avoid common pitfalls when changing the RELEASE variable

Fixes RPi-Distro#635, fixes RPi-Distro#645, fixes RPi-Distro#632

Author: XECDesign

README.md

@@ -27,10 +27,10 @@ Getting started is as simple as cloning this repository on your build machine. Y
 can do so with:
 
 ```bash
-git clone --depth 1 https://github.com/RPI-Distro/pi-gen.git
+git clone https://github.com/RPI-Distro/pi-gen.git
 ```
 
-Using `--depth 1` with `git clone` will create a shallow clone, only containing
+`--depth 1` can be added afer `git clone` to create a shallow clone, only containing
 the latest revision of the repository. Do not do this on your development machine.
 
 Also, be careful to clone the repository to a base path **NOT** containing spaces.
@@ -80,8 +80,11 @@ The following environment variables are supported:
 
 * `RELEASE` (Default: bullseye)
 
-   The release version to build images against. Valid values are jessie, stretch,
-   buster, bullseye, and testing.
+   The release version to build images against. Valid values are any supported
+   Debian release. However, since different releases will have different sets of
+   packages available, you'll need to either modify your stages accordingly, or
+   checkout the appropriate branch. For example, if you'd like to build a
+   `buster` image, you should do so from the `buster` branch.
 
  * `APT_PROXY` (Default: unset)
 
@@ -489,7 +492,7 @@ A 64 bit image can be generated from the `arm64` branch in this repository. Just
 replace the command from [this section](#getting-started-with-building-your-images)
 by the one below, and follow the rest of the documentation:
 ```bash
-git clone --depth 1 --branch arm64 https://github.com/RPI-Distro/pi-gen.git
+git clone --branch arm64 https://github.com/RPI-Distro/pi-gen.git
 ```
 
 If you want to generate a 64 bits image from a Raspberry Pi running a 32 bits

build.sh

@@ -226,7 +226,7 @@ export TARGET_HOSTNAME=${TARGET_HOSTNAME:-raspberrypi}
 export FIRST_USER_NAME=${FIRST_USER_NAME:-pi}
 export FIRST_USER_PASS
 export DISABLE_FIRST_BOOT_USER_RENAME=${DISABLE_FIRST_BOOT_USER_RENAME:-0}
-export RELEASE=${RELEASE:-bullseye}
+export RELEASE=${RELEASE:-bullseye} # Don't forget to update stage0/prerun.sh
 export WPA_ESSID
 export WPA_PASSWORD
 export WPA_COUNTRY

stage0/prerun.sh

@@ -1,5 +1,10 @@
 #!/bin/bash -e
 
+if [ "$RELEASE" != "bullseye" ]; then
+	echo "WARNING: RELEASE does not match the intended option for this branch."
+	echo "         Please check the relevant README.md section."
+fi
+
 if [ ! -d "${ROOTFS_DIR}" ] || [ "${USE_QCOW2}" = "1" ]; then
 	bootstrap ${RELEASE} "${ROOTFS_DIR}" http://raspbian.raspberrypi.org/raspbian/
 fi