Use custom Dockerfile for GitHub Codespaces

src/.devcontainer/Dockerfile

@@ -0,0 +1,12 @@
+FROM mcr.microsoft.com/devcontainers/universal:linux
+
+RUN mkdir -p /opt/gcc && \
+    cd /opt/gcc && \
+    wget -q https://developer.arm.com/-/media/Files/downloads/gnu-a/10.3-2021.07/binrel/gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf.tar.xz && \
+    tar xf gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf.tar.xz && \
+    rm gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf.tar.xz && \
+    wget -q https://developer.arm.com/-/media/Files/downloads/gnu-a/10.3-2021.07/binrel/gcc-arm-10.3-2021.07-x86_64-arm-none-eabi.tar.xz && \
+    tar xf gcc-arm-10.3-2021.07-x86_64-arm-none-eabi.tar.xz && \
+    rm gcc-arm-10.3-2021.07-x86_64-arm-none-eabi.tar.xz
+
+ENV PATH=/opt/gcc/gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf/bin:/opt/gcc/gcc-arm-10.3-2021.07-x86_64-arm-none-eabi/bin:$PATH

src/.devcontainer/devcontainer.json

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+{
+  "build": { "dockerfile": "Dockerfile" },
+}