readme: add docker instructions

readme.md

···
 
 There's a couple of ways to get started:
 * NixOS: refer to [flake.nix](https://tangled.sh/@tangled.sh/core/blob/master/flake.nix)
+* Docker: Documented below.
 * Manual: Documented below.
+
+## docker setup
+
+Clone this repository:
+
+```
+git clone https://tangled.sh/@tangled.sh/core
+```
+
+Modify the `docker/docker-compose.yml`, specifically the
+`KNOT_SERVER_SECRET` and `KNOT_SERVER_HOSTNAME` env vars. Then run:
+
+```
+docker compose -f docker/docker-compose.yml up
+```
 
 ### manual setup