Docker build env for windows/android

docker/build-env/README.md

@@ -0,0 +1,56 @@
+# Build environments
+
+Dockerfiles that pin the same toolchain CI uses, so a developer can
+reproduce a CI build locally without installing platform SDKs on their
+workstation. The version pins in each `Dockerfile` must stay in lockstep
+with `.github/workflows/`.
+
+## `android/`
+
+Mirrors `.github/workflows/mobile-build-validation.yml` (`android_build`
+job). Carries Go 1.25.5, Adopt JDK 11, Android cmdline-tools 8512546,
+NDK 23.1.7779620 and gomobile pinned at the CI commit. Use it to
+produce `netbird.aar` from `./client/android`:
+
+```bash
+docker build -t netbird/build-android docker/build-env/android
+docker run --rm -v "$PWD:/src" -w /src netbird/build-android \
+    gomobile bind \
+    -o netbird.aar \
+    -javapkg=io.netbird.gomobile \
+    -ldflags="-checklinkname=0 \
+              -X golang.zx2c4.com/wireguard/ipc.socketDirectory=/data/data/io.netbird.client/cache/wireguard \
+              -X github.com/netbirdio/netbird/version.version=local" \
+    ./client/android
+```
+
+To build the full Android APK, bind-mount the `android-client` repo as
+well and run its own `./gradlew assembleDebug` from inside the
+container (the gradle wrapper ships with `android-client`).
+
+## `windows-cross/`
+
+Cross-compiles Windows binaries from Linux using `mingw-w64`. Lets you
+verify that `GOOS=windows go build ./...` compiles cleanly without
+needing a Windows VM. Cannot run Windows tests — the `golang-test-windows`
+CI job executes on a native `windows-latest` runner with wintun.dll
+and PsExec, neither of which lives under Linux containers.
+
+```bash
+docker build -t netbird/build-windows docker/build-env/windows-cross
+docker run --rm -v "$PWD:/src" -w /src netbird/build-windows \
+    bash -c 'GOOS=windows GOARCH=amd64 go build ./...'
+```
+
+## What is NOT here
+
+- **iOS / macOS**: cannot legally run macOS in Docker (Apple EULA),
+  and Xcode is not redistributable. The `ios_build` CI job uses a
+  `macos-latest` GitHub runner; locally you need a real Mac.
+
+- **Native Windows tests**: see note above. The Linux+mingw image
+  builds, it does not execute Windows-host code paths
+  (registry, wintun, services, PsExec workflows).
+
+When CI version pins change, update the corresponding `ARG` lines in
+the Dockerfiles and the README's table of versions.