Add docs build readme

docs/build/README.md

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+## Overview
+
+Prebid Server contains at least one module that requires CGo which introduces both build and runtime dependencies.
+To build, you need a C compiler, preferably gcc.
+To run, you may require one or more runtime dependencies, most notably libatomic.
+
+## Examples (Build --> Target)
+For a containerized example, see the Dockerfile.
+Here are some manual build examples, including some cross-compilation use cases, that have been tested:
+
+### darwin amd64 --> darwin amd64
+`GOOS=darwin GOARCH=amd64 CGO_ENABLED=1 go build`
+
+Running the built binary on mac amd64:
+`./prebid-server --stderrthreshold=WARNING -v=2`
+
+### darwin amd64 --> darwin arm64
+`GOOS=darwin GOARCH=arm64 CGO_ENABLED=1 go build`
+
+Running the built binary on mac arm64:
+`./prebid-server --stderrthreshold=WARNING -v=2`
+
+### darwin amd64 --> windows amd64
+<b>Build (mac)</b>
+Install mingw-w64 which consists of a gcc compiler port you can use to generate windows binaries:
+`brew install mingw-w64`
+
+`GOOS=windows GOARCH=amd64 CGO_ENABLED=1 CC="x86_64-w64-mingw32-gcc" go build`
+
+<b>Run (windows)</b>
+Running the built binary on windows:
+`.\prebid-server.exe --sderrthreshold=WARNING =v=2`
+
+You may receive the following errors or something similar:
+```
+"The code execution cannot proceed because libatomic-1.dll was not found."
+"The code execution cannot proceed because libwinpthread-1.dll was not found."
+```
+
+To resolve these errors, copy the following files from mingw-64 on your mac to `C:/windows/System32` and re-run:
+`/usr/local/Cellar/mingw-w64/12.0.0_1/toolchain-x86_64/x86_64-w64-mingw32/lib/libatomic-1.dll`
+`/usr/local/Cellar/mingw-w64/12.0.0_1/toolchain-x86_64/x86_64-w64-mingw32/bin/libwinpthread-1.dll`
+
+### windows amd64 --> windows amd64
+<b>Build</b>
+`set CGO_ENABLED=1`
+`set GOOS=windows`
+`set GOARCH=amd64`
+`go build . && .\prebid-server.exe --stderrthreshold=WARNING -v=2`
+
+You may receive the following error or something similar:
+```
+# runtime/cgo
+cgo: C compiler "gcc" not found: exec: "gcc": executable file not found in %PATH%
+```
+
+To resolve the error, install MSYS2:
+1) Download the installer (https://www.msys2.org/)
+2) Run the installer and follow the steps of the installation wizard
+3) Run MSYS2 which will open an MSYS2 terminal for you
+4) In the MSYS2 terminal, install windows/amd64 gcc toolchain: `pacman -S --needed base-devel mingw-w64-x86_64-gcc`
+5) Enter `Y` when prompted whether to proceed with the installation
+6) Add the path of your MinGW-w64 `bin` folder to the Windows `PATH` environment variable by using the following steps:
+    - In the Windows search bar, type <b>Settings</b> to open your Windows Settings.
+    - Search for <b>Edit environment variables for your account</b>.
+    - In your <b>User variables</b>, select the `Path` variable and then select <b>Edit</b>.
+    - Select </b>New and add the MinGW-w64 destination folder you recorded during the installation process to the list. If you used the default settings above, then this will be the path: `C:\msys64\ucrt64\bin`.
+    - Select <b>OK</b>, and then select <b>OK</b> again in the <b>Environment Variables</b> window to update the `PATH` environment variable. You have to reopen any console windows for the updated `PATH` environment variable to be available.
+7) Confirm gcc installed: `gcc --version`
+
+<b>Run</b>
+Running the built binary on windows:
+`go build . && .\prebid-server.exe --stderrthreshold=WARNING -v=2`
+
+You may receive the following errors or something similar:
+```
+"The code execution cannot proceed because libatomic-1.dll was not found."
+"The code execution cannot proceed because libwinpthread-1.dll was not found."
+```
+To resolve these errors, copy the following files from MSYS2 installation to `C:/windows/System32` and re-run:
+`C:\mysys64\mingw64\bin\libatomic-1.dll`
+`C:\mysys64\mingw64\bin\libwinpthread-1.dll`
+
+### linux amd64 --> linux amd64
+<b>Build</b>
+`GOOS=linux GOARCH=amd64 CGO_ENABLED=1 go build`
+
+You may receive the following error or something similar:
+```
+# runtime/cgo
+cgo: C compiler "gcc" not found: exec: "gcc": executable file not found in $PATH
+```
+To resolve the error, install gcc and re-build:
+`sudo apt-get install -y gcc`
+
+<b>Run</b>
+Running the built binary on Linux:
+`./prebid-server --stderrthreshold=WARNING -v=2`
+
+You may receive the following error or something similar:
+```
+... error while loading shared libraries: libatomic.so.1: cannot open shared object file ...
+```
+To resolve the error, install libatomic1 and re-run:
+`sudo apt-get install -y libatomic1`