Expand DEVELOPERS docs

DEVELOPERS.md

@@ -1,9 +1,13 @@
 # Notes for developers
 
 
-## Local development environment
+## Prerequisites for local development
 
-We use [just](https://github.com/casey/just) as our command runner.
+### Just
+
+We use [`just`](https://github.com/casey/just) as our command runner. It's
+a single file binary available for many platforms so should be easy to
+install.
 
 ```sh
 # macOS
@@ -19,7 +23,53 @@ source <(just --completions bash)
 just #  shortcut for just --list
 ```
 
+### Python
+
+You'll need an appropriate version of Python on your PATH. Check the
+`.python-version` file for the required version.
+
+### Docker
+
+Possibly something API compatibile with Docker would also work, but we
+don't officially support that.
+
+
+## Getting started
+
 Set up a local development environment with:
 ```
 just devenv
 ```
+
+You'll probably find you need to run this twice. The first time it will
+complain that you don't have a `.env` environment file and will create
+one for you. Then you'll need to run the command again with the `.env`
+file in place.
+
+Check that Django is configured correctly with:
+```
+just manage check
+```
+
+You can run all the tests with:
+```
+just test
+```
+
+### Running commands without using `just`
+
+`just` automatically takes care of a few things:
+
+ * ensuring commands are run using the correct Python virtual
+   environment;
+ * ensuring that the installed packages match what's specified in the
+   `requirements.*.txt` files;
+ * ensuring that the variables specified in `.env` are loaded into the
+   environment.
+
+Running commands outside of `just` is a reasonable and supported
+workflow, but you will need to handle the above tasks yourself e.g. by
+activating a virtual environment and running something like:
+```bash
+set -a; source .env; set +a
+```

justfile

@@ -40,8 +40,13 @@ virtualenv: ensure-env
 
     # create venv and upgrade pip
     if [[ ! -d $VIRTUAL_ENV ]]; then
+      # Collapse output when running in Github Actions
+      [[ -v CI ]] && echo "::group::Setting up venv (click to view)" || true
+
       $PYTHON_VERSION -m venv $VIRTUAL_ENV
       $PIP install --upgrade pip
+
+      [[ -v CI ]]  && echo "::endgroup::" || true
     fi