docker build instructions

build.md

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+## Compilation
+
+Install `Go >= 1.16` and `GCC`. Get the source code:
+
+    git clone https://github.com/nkanaev/yarr.git
+
+Then run one of the corresponding commands:
+
+    # create an executable for the host os
+    make build_macos    # -> _output/macos/yarr.app
+    make build_linux    # -> _output/linux/yarr
+    make build_windows  # -> _output/windows/yarr.exe
+
+    # host-specific cli version (no gui)
+    make build_default  # -> _output/yarr
+
+    # ... or start a dev server locally
+    make serve          # starts a server at http://localhost:7070
+
+    # ... or build a docker image
+    docker build -t yarr .
+
+## ARM compilation
+
+The instructions below are to cross-compile *yarr* to `Linux/ARM*`.
+
+Build:
+
+    docker build -t yarr.arm -f dockerfile.arm .
+
+Test:
+
+    # inside host
+    docker run -it --rm yarr.arm
+
+    # then, inside container
+    cd /root/out
+    qemu-aarch64 -L /usr/aarch64-linux-gnu/ yarr.arm64
+
+Extract files from images:
+
+    CID=$(docker create yarr.arm)
+    docker cp -a "$CID:/root/out" .
+    docker rm "$CID"