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

HACKING.md

@@ -0,0 +1,42 @@
+# Hacking on image-builder-cli
+
+Hacking on `image-builder` should be fun and is easy.
+
+We have unit tests and some integration testing.
+
+## Setup
+
+To work on bootc-image-builder one needs a working Go environment. See
+[go.mod](go.mod).
+
+To run the testsuite install the test dependencies as outlined in the
+[github action](./.github/workflows/go.yml) under
+"Install test dependencies".
+
+## Code layout
+
+The go source code of image-builder-cli is under
+`./cmd/image-builder`. It uses the
+[images](https://github.com/osbuild/images) library internally to
+generate the images. Unit tests (and integration tests where it makes
+sense) are expected to be part of every PR but we are happy to help if
+those are missing from a PR.
+
+## Build
+
+Build by running:
+```console
+$ go build ./cmd/image-builder/
+```
+
+## Unit tests
+
+Run the unit tests via:
+```console
+$ go test -short ./...
+```
+
+There are some integration tests that can be run via:
+```console
+$ go test ./...
+```