✅ tests: add justfile to run tests and documentation

Author: edouard-lopez

README.md

@@ -3,3 +3,11 @@
 ```console
 yarn install
 ```
+
+## Tests
+
+Pass a feature and a base image, _e.g._:
+
+```fish
+just test fish mcr.microsoft.com/devcontainers/base:debian
+```

justfile

@@ -0,0 +1,8 @@
+set shell := ["fish", "-c"]
+
+
+test feature image:
+  devcontainer features test \
+    --skip-scenarios \
+    --features {{feature}} \
+    --base-image {{image}} .