Move description to create a dual host builder instance to CONTRIBUTING.md

And add a buildkit config which disables the garbage collection of the cache.
Our images are too many and too fat, so that it gets deleted half way otherwise.

Author: larskanis

CONTRIBUTING.md

@@ -53,3 +53,46 @@ docker buildx create --use --driver=docker-container
 bundle exec rake build
 ```
 
+
+### 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 (~45 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
+# Disable the garbage collector by the config file
+$ docker buildx create --name isayoga --config build/buildkitd.toml
+
+# Add the remote instance
+$ docker buildx create --name isayoga --config build/buildkitd.toml --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
+```

build/buildkitd.toml

@@ -0,0 +1,2 @@
+[worker.oci]
+  gc = false

mingw64-ucrt/README.md

@@ -12,47 +12,3 @@ 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
-```