Merge pull request #2 from a3nm/master

Document apt-get workaround

README.md

@@ -9,12 +9,39 @@ See https://wiki.debian.org/RaspberryPi3#Preview_image for where to obtain the l
 
 ## Option 2: Building your own image
 
-If you prefer, you can build a Debian buster Raspberry Pi 3 image using:
+If you prefer, you can build a Debian buster Raspberry Pi 3 image yourself. For
+this, first install the
+[requirements](https://github.com/larswirzenius/vmdb2/blob/master/README#getting-vmdb2)
+of vmdb2. Then run the following:
 
 ```shell
 git clone --recursive https://github.com/Debian/raspi3-image-spec
 cd raspi3-image-spec
-sudo ./vmdb2/vmdb2 --output raspi3.img raspi3.yaml --log raspi3.log
+```
+
+Then edit [raspi3.yaml](raspi3.yaml) to select the Debian repository that you
+want to use:
+
+- If you want to use the snapshot with which the build was tested, use
+    `http://snapshot.debian.org/archive/debian/20171007T213914Z/`. This is what
+    is pre-configured in raspi3.yaml. However, due to a [missing
+    feature](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=763419) on
+    snapshots, to make the build work, you have to disable an expiration check
+    by APT. To do so, edit raspi3.yaml to replace all
+    `apt-get` invocations with `apt-get -o Acquire::Check-Valid-Until=false`
+- If you want to use the latest versions of each software, you can replace
+    `http://snapshot.debian.org/archive/debian/20171007T213914Z/` in raspi3.yaml
+    with `http://deb.debian.org/debian`. Of course, this means that the
+    build may break or fail to boot if there are regressions in the latest
+    versions.
+
+Once you have edited raspi3.yaml, you can generate the image by
+issuing the following:
+
+```shell
+umask 022
+sudo env -i LC_CTYPE=C.UTF-8 PATH="$PATH" \
+    ./vmdb2/vmdb2 --output raspi3.img raspi3.yaml --log raspi3.log
 ```
 
 ## Installing the image onto the Raspberry Pi 3
@@ -36,6 +63,3 @@ ssh root@rpi3
 # Enter password “raspberry”
 ```
 
-## Reproducibility
-
-The image currently uses http://snapshot.debian.org/archive/debian/20171007T213914Z/ for ensuring a reproducible build.