Add the base containerized c toolchain environment

TOOLCHAIN.md

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+# Instruction on running utilities
+
+## c_toolchain utility docker image
+
+Dockerfile: `dockerfile.c_toolchain`
+Run the docker-compose command to get a `bash` shell that is bound to the `dmzOS` directory:
+```bash
+docker-compose run --rm c_toolchain
+```
+
+Any changes in the container shell will be reflected in the local `dmzOS` directory.
+
+The image contains `ident` abd `clang-format` as `C` code formatters. A .vimrc file is automatically part of the image, and it contains a rule to make vim run the following formatting command on buffer save:
+```bash
+ident -kr -ts4 %
+```
+
+This is using the Kernighan and Ritchie code styling, as well as identations with 4 spaces.