Add Docker section to README (#1791)

* Add Docker section to README

Added information on moonlight-packaging to building section of README.

Co-authored-by: Cameron Gutman <aicommander@gmail.com>

Author: sygi

README.md

@@ -82,6 +82,10 @@ Moonlight builds for Steam Link are subject to hardware limitations of the Steam
 * Maximum video bitrate: **40 Mbps**
 * **HDR streaming is not supported** on the original hardware
 
+### Docker containers
+If you want to use Docker for building, look at [this repo](https://github.com/cgutman/moonlight-packaging) containing canonical containers
+for different architectures, which handle building deps and extra linking for you.
+
 ### Build Setup Steps
 1. Install the latest Qt SDK (and optionally, the Qt Creator IDE) from https://www.qt.io/download
     * You can install Qt via Homebrew on macOS, but you will need to use `brew install qt --with-debug` to be able to create debug builds of Moonlight.
@@ -93,7 +97,8 @@ Moonlight builds for Steam Link are subject to hardware limitations of the Steam
         * For Windows builds, use `scripts\build-arch.bat` and `scripts\generate-bundle.bat`. Execute these scripts from the root of the repository within a Qt command prompt. Ensure  7-Zip binary directory is on your `%PATH%`.
         * For macOS builds, use `scripts/generate-dmg.sh`. Execute this script from the root of the repository and ensure Qt's `bin` folder is in your `$PATH`.
         * For Steam Link builds, run `scripts/build-steamlink-app.sh` from the root of the repository.
-    * To build from the command line for development use on macOS or Linux, run `qmake6 moonlight-qt.pro` then `make debug` or `make release`
+    * To build from the command line for development use on macOS or Linux, run `qmake6 moonlight-qt.pro` then `make debug` or `make release`.
+        * The final binary will be placed in `app/moonlight`.
     * To create an embedded build for a single-purpose device, use `qmake6 "CONFIG+=embedded" moonlight-qt.pro` and build normally.
         * This build will lack windowed mode, Discord/Help links, and other features that don't make sense on an embedded device.
         * For platforms with poor GPU performance, add `"CONFIG+=gpuslow"` to prefer direct KMSDRM rendering over GL/Vulkan renderers. Direct KMSDRM rendering can use dedicated YUV/RGB conversion and scaling hardware rather than slower GPU shaders for these operations.