Multiplatform docker build

[lmandrelli]

This PR updates the github actions workflow to build the container both for amd64 and arm64 platforms.

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Why is it necessary ?

I tried to install the MCP on my Mac, after having copied the configuration for opencode, I was surprised to see that it failed executing. Pulling manually the image showed me that the image was only built for amd64 architecture.

This cloud be overcomed by adding --platform=linux/amd64 to the mcp command, but it comes with two problems :

• specific configuration to document for apple silicon macs
• non native execution

Proposed solution with this PR

I updated the github actions workflow to build on a matrix of 2 machines
The workflow now does 2 base image build, wait for these two builds to complete and merge result in a single node, then build the two runtime images, then merge results and push the images for both amd64 and arm64 platforms.

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Tests

I tested these builds on my fork. You can find the images built for both architecture there. I tested the two images on my computer.
You can now use the normal configuration for opencode without friction.