Add a README for creating the mingw64-ucrt docker image

Author: larskanis

mingw64-ucrt/README.md

@@ -0,0 +1,58 @@
+Docker image with compilers for ruby platform x64-mingw-ucrt
+------------------
+
+This Dockerfile builds compilers for Windows UCRT target.
+It takes the mingw compiler provided by Debian/Ubuntu and configures and compiles them for UCRT.
+Outputs are *.deb files of binutils, gcc and g++.
+Rake-compiler-dock reads them from this image as part of its build process for the x64-mingw-ucrt platform.
+
+The image is provided for arm64 and amd64 architectures.
+They are built by the following command:
+
+```sh
+docker buildx build . -t larskanis/mingw64-ucrt:20.04 --platform linux/arm64,linux/amd64 --push
+```
+
+
+Create builder instance for two architectures
+------------------
+
+Building with qemu emulation fails currently with a segfault, so that it must be built by a builder instance with at least one remote node for the other architecture.
+Building on native hardware is also much faster (~30 minutes) than on qemu.
+A two-nodes builder requires obviously a ARM and a Intel/AMD device.
+It can be created like this:
+
+```sh
+# Make sure the remote instance can be connected
+$ docker -H ssh://isa info
+
+# Create a new builder with the local instance
+$ docker buildx create --name isayoga
+
+# Add the remote instance
+$ docker buildx create --name isayoga --append ssh://isa
+
+# They are inactive from the start
+$ docker buildx ls
+NAME/NODE      DRIVER/ENDPOINT                   STATUS     BUILDKIT   PLATFORMS
+isayoga        docker-container
+ \_ isayoga0    \_ unix:///var/run/docker.sock   inactive
+ \_ isayoga1    \_ ssh://isa                     inactive
+default*       docker
+ \_ default     \_ default                       running    v0.13.2    linux/arm64
+
+# Bootstrap the instances
+$ docker buildx inspect --bootstrap --builder isayoga
+
+# Set the new builder as default
+$ docker buildx use isayoga
+
+# Now it should be default and in state "running"
+$ docker buildx ls
+NAME/NODE      DRIVER/ENDPOINT                   STATUS    BUILDKIT   PLATFORMS
+isayoga*       docker-container
+ \_ isayoga0    \_ unix:///var/run/docker.sock   running   v0.18.2    linux/arm64
+ \_ isayoga1    \_ ssh://isa                     running   v0.18.2    linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/386
+default        docker
+ \_ default     \_ default                       running   v0.13.2    linux/arm64
+```