Ensure devcontainer uses the platform of the toolchain (#1073)

* Refactor devcontainer into Docker Compose

We need to make the platform explicit (only via the Docker Compose) as the Dockerfile fetches a crosscompiler for x86.

* Add git to dev container

.devcontainer/devcontainer.json

@@ -1,5 +1,5 @@
 {
-  "build": {
-    "dockerfile": "Dockerfile"
-  }
-}
+  "dockerComposeFile": "docker-compose.yml",
+  "service": "devcontainer",
+  "workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}"
+}