HACKING.md: add basic documentation for the development

This commit adds a basic HACKING.md that explains the bare mimimum
to get started, including `go test -short` to skip the expensive
tests.

Contains also a drive-by rename of the test dependency install
for the GH action.

.github/workflows/go.yml

@@ -25,7 +25,7 @@ jobs:
       run: sudo apt update
 
     # This is needed for the container resolver dependencies
-    - name: Install libgpgme devel package
+    - name: Install test dependencies
       run: sudo apt install -y libgpgme-dev libbtrfs-dev libdevmapper-dev podman
 
     - name: Build