Recommend Docker have 5GB memory available.

Co-authored-by: Alan Evans <alan@signal.org>

reproducible-builds/README.md

@@ -134,6 +134,13 @@ Your platform might also have its own preferred way of installing Docker. E.g. U
 
 In the following sections we will assume that your Docker installation works without issues. So after installing, please make sure that everything is running smoothly before continuing.
 
+### Configuring Docker runtime memory
+
+Docker seems to require at least 5GB runtime memory to be able to build the APK successfully. Docker behaves differently on each platform - please consult Docker documentation for the platform of choice to configure the runtime memory settings.
+
+ * https://docs.docker.com/config/containers/resource_constraints/
+ * OS X https://docs.docker.com/docker-for-mac/#resources
+ * Windows https://docs.docker.com/docker-for-windows/#resources
 
 ## Building a Docker image for Signal
 First, you need to pull down the source for Signal-Android, which contains everything you need to build the project, including the `Dockerfile`. The `Dockerfile` contains instructions on how to automatically build a Docker image for Signal. It's located in the `reproducible-builds` directory of the repository. To get it, clone the project: