ci: test build Dockerfile

test/Dockerfile

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+# syntax=docker/dockerfile:1
+
+FROM --platform=$BUILDPLATFORM golang:alpine AS build
+ARG TARGETPLATFORM
+ARG BUILDPLATFORM
+RUN echo "I am running on $BUILDPLATFORM, building for $TARGETPLATFORM" > /log
+
+FROM alpine
+COPY --from=build /log /log

test/hello.txt

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+
+Hello from Docker! 
+This message shows that your installation appears to be working correctly.
+
+To generate this message, Docker took the following steps:
+ 1. The Docker client contacted the Docker daemon.
+ 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
+    (windows-amd64, nanoserver-1809)
+ 3. The Docker daemon created a new container from that image which runs the
+    executable that produces the output you are currently reading.
+ 4. The Docker daemon streamed that output to the Docker client, which sent it
+    to your terminal.
+
+To try something more ambitious, you can run a Windows Server container with:
+ PS C:\> docker run -it mcr.microsoft.com/windows/servercore:ltsc2019 powershell
+
+Share images, automate workflows, and more with a free Docker ID:
+ https://hub.docker.com/
+
+For more examples and ideas, visit:
+ https://docs.docker.com/get-started/
+

test/win.Dockerfile

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+FROM mcr.microsoft.com/windows/nanoserver:1809
+COPY hello.txt C:
+CMD ["cmd", "/C", "type C:\\hello.txt"]