README: Update build instructions

Update the build instructions with
steps for containerized builds.

Changelog-entry: Update build instructions in README
Signed-off-by: Alex Gonzalez <alexg@balena.io>

Author: alexgg

README.md

@@ -10,24 +10,36 @@ bring in all the needed dependencies.
 
 ## Build information
 
-### Build flags
+balenaOS currently only builds with cgroups v1. If your distribution defaults
+to using cgroups v2, please boot with the following kernel command line
+argument:
+`systemd.unified_cgroup_hierarchy=0`
 
-* Consult layers/meta-balena/README.md for info on various build flags (setting
-up serial console support for example) and build prerequisites. Build flags can
-be set by using the build script (barys) or by manually modifying `local.conf`.
+### Containerized build
+
+* If you have a working docker installation, you can build in a containerized
+  environment as follows:
+  `./balena-yocto-scripts/build/balena-build.sh -d <device type> -s <shared directory>`
+
+  Where:
+    * Device type is one of the supported devices with a valid `<device type name>.coffee` description file.
+    * Shared directory is the absolute path to the build folder
+
+### Native build
 
-See below for using the build script.
+To build all supported device types natively, please make sure your Linux
+distribution is [supported](https://docs.yoctoproject.org/singleindex.html#supported-linux-distributions) by Yocto Project.
 
-### Build this repository
+Additional host tools need to be installed for native builds to work.
 
-* Run the build script:
+* Run the barys build script:
   `./balena-yocto-scripts/build/barys`
 
 * You can also run barys with the -h switch to inspect the available options
 
 ### Custom build using this repository
 
-* Run the build script in dry run mode to setup an empty `build` directory
+* Run the barys build script in dry run mode to setup an empty `build` directory
     `./balena-yocto-scripts/build/barys --remove-build --dry-run`
 
 * Edit the `local.conf` in the `build/conf` directory
@@ -37,6 +49,13 @@ See below for using the build script.
 
 * Run bitbake (see message outputted when you sourced above for examples)
 
+### Build flags
+
+* Consult layers/meta-balena/README.md for info on various build flags (setting
+up serial console support for example) and build prerequisites. Build flags can
+be set by using the build scripts (barys or balena-build) or by manually
+modifying `local.conf`.
+
 ## Contributing
 
 ### Issues