Update readme

README.md

@@ -1,18 +1,58 @@
 # arm-runner-action
-GitHub action to run CI within a virtualized ARM environment, typically
-raspbian.
+
+Run tests natively and build images directly from GitHub Actions using a
+chroot-based virtualized Raspberry Pi (raspios/raspbian) environment.
+
+With this action, you can:
+- run tests in an environment closer to a real embedded system, using qemu
+userland linux emulation;
+- build artifacts in such environment and upload them;
+- prepare images that are ready to run on Raspberry-Pi and other arm embedded
+devices.
 
 ## Usage
 
 Minimal usage is as follows:
 
     jobs:
       build:
+        runs-on: ubuntu-latest
+        steps:
+        - uses: actions/checkout@v2
         - uses: pguyot/arm-runner-action@v1
           with:
             commands: |
-                some commands
+                commands to run tests
 
+Typical usage to upload an image as an artifact:
+
+    jobs:
+      build:
+        runs-on: ubuntu-latest
+        steps:
+        - uses: actions/checkout@v2
+        - uses: pguyot/arm-runner-action@v1
+          id: build_image
+          with:
+            commands: |
+                commands to build image
+        - name: Compress the release image
+          if: github.ref == 'refs/heads/releng' || startsWith(github.ref, 'refs/tags/')
+          run: |
+            mv ${{ steps.build_image.outputs.image }} my-release-image.img
+            xz -0 -T 0 -v my-release-image.img
+        - name: Upload release image
+          uses: actions/upload-artifact@v2
+          if: github.ref == 'refs/heads/releng' || startsWith(github.ref, 'refs/tags/')
+          with:
+            name: Release image
+            path: my-release-image.img.xz
+
+### Commands
+
+The repository is copied to the image before the commands script is executed
+in the chroot environment. The commands script is copied to /tmp/ and is
+deleted on cleanup.
 
 ### Inputs
 
@@ -28,7 +68,7 @@ The following values are allowed:
 - `raspbian_lite:2020-02-13`
 - `raspios_lite:2021-03-04` (default)
 
-More images will be added, eventually.
+More images will be added, eventually. Feel free to submit PRs.
 
 #### `image_additional_mb`
 
@@ -42,13 +82,24 @@ Other values include `cortex-a8` which translates to arm7vl.
 
 #### `copy_artifact_path`
 
-Source path to copy outside the image. Relative to the working directory, within the
-image. Globs are allowed. Default is not to copy.
+Source path to copy outside the image. Relative to the working directory,
+within the image. Globs are allowed. Default is not to copy.
 
 #### `copy_artifact_dest`
 
 Destination path to copy outside the image. Relative to the working directory
-(outside the image). Default to `.`
+(outside the image). Defaults to `.`
+
+#### `repository_path`
+
+Absolute path, inside the image, where the repository is copied. Defaults
+to `/<repository_name>`. It is also the working directory where commands are
+executed.
+
+#### `use_systemd_nspawn`
+
+Use `systemd-nspanw` instead of chroot to run commands. Default is to use
+chroot.
 
 ### Outputs