Add Containerfile for development

Add a command to create a Debian-based container with the coreboot
toolchains, SDCC and Rust installed in order to minimize the setup time
of a container-based workflow, such as CI.

    make -C containers
    podman run -it --rm \
        -v $PWD:/workspace:Z \
        -v ~/.ccache:/root/.ccache:Z \
        system76/firmware-open:latest \
        ./scripts/build.sh oryp8

A locally built image is 2.86 GiB.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

containers/Makefile

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
+# Disable built-in rules and variables
+MAKEFLAGS += --no-builtin-rules --no-builtin-variables
+.SUFFIXES:
+
+# Default to silent builds
+ifneq ($(VERBOSE),1)
+MAKEFLAGS += --silent
+.SILENT:
+endif
+
+PODMAN := $(shell command -v podman)
+
+CONTAINER_TAG := latest
+
+.PHONY: firmware-open
+firmware-open:
+	$(PODMAN) build \
+		--tag system76/$@:$(CONTAINER_TAG) \
+		--file Containerfile \
+		$@