Containerfile: update

[s373r]

Context:

• Once again, I decided to try experimenting with fan curves on my addw4, I noticed that there is a Containerfile, which is very handy
• I needed to modify the scripts a bit to achieve parity with the native building (and fix cargo not found error)

Can now be called with passing custom coreboot revision parameters:

COREBOOT_REPO=https://github.com/system76/coreboot \
COREBOOT_COMMIT=system76 \
	make -C containers