Use online checksums for Chisel tool in .NET SDK Dockerfiles

[lbussell]

This is part of #6548. Instead of using hard-coded checksums, they should be downloaded from the Chisel github release where they exist as .sha384 files. This makes the Dockerfiles more flexible and simplifies this repo's maintenance, since we will no longer need to calculate or keep track of checksums.

Example:

dotnet-docker/documentation/ubuntu-chiseled.md

Lines 64 to 70 in 157267f

	# Install chisel and chisel-wrapper
	RUN chisel_url=https://github.com/canonical/chisel/releases/download/v${CHISEL_VERSION}/chisel_v${CHISEL_VERSION}_linux_amd64.tar.gz \
	&& curl -fSLOJ ${chisel_url} \
	&& curl -fSL ${chisel_url}.sha384 | sha384sum -c - \
	&& tar -xzf chisel_v${CHISEL_VERSION}_linux_amd64.tar.gz -C /usr/bin/ chisel \
	&& curl -fSL --output /usr/bin/chisel-wrapper https://raw.githubusercontent.com/canonical/rocks-toolbox/${CHISEL_WRAPPER_VERSION}/chisel-wrapper \
	&& chmod 755 /usr/bin/chisel-wrapper