Update INSTALL.md to reference Docker as an option

INSTALL.md

@@ -3,12 +3,17 @@
 These instructions are designed for setting up The Rails Port for development and testing.
 If you want to deploy the software for your own project, then see the notes at the end.
 
-You can install the software directly on your machine, which is the traditional and probably best-supported approach. However, there is an alternative which may be easier: Vagrant. This installs the software into a virtual machine, which makes it easier to get a consistent development environment and may avoid installation difficulties. For Vagrant instructions, see [VAGRANT.md](VAGRANT.md).
+You can install the software directly on your machine, which is the traditional and probably best-supported approach. However, there
+are two alternatives which make it easier to get a consistent development environment and may avoid installation difficulties:
+
+* **Vagrant** This installs the software into a virtual machine. For Vagrant instructions see [VAGRANT.md](VAGRANT.md).
+* **Docker** This installs the software using containerization. For Docker instructions see [DOCKER.md](DOCKER.md).
 
 These instructions are based on Ubuntu 20.04 LTS, which is the platform used by the OSMF servers.
 The instructions also work, with only minor amendments, for all other current Ubuntu releases, Fedora and MacOSX
 
-We don't recommend attempting to develop or deploy this software on Windows. If you need to use Windows, then try developing this software using Ubuntu in a virtual machine, or use [Vagrant](VAGRANT.md).
+These instructions are based on Ubuntu 18.04 LTS, which is the platform used by the OSMF servers.
+The instructions also work, with only minor amendments, for all other current Ubuntu releases, Fedora and MacOSX.
 
 ## Dependencies