Add support for RISC-V

[chazapis]

Since Helm does not provide riscv64 binaries yet, I use a separate Dockerfile that builds from source. I have already submitted a PR to Helm for releasing a RISC-V binary. When it is merged, the Dockerfile.riscv64 can be simplified.

The entry script has also been modified to function without Helm v2. Helm v2 is deprecated, a release has not been made for close to 3 years, and according to the documentation v2 charts can also be installed with Helm v3. The new entry script should function even when Helm v2 is required, but can also be easily modified to fail with an error.

The work is part of an ongoing effort to run K3s on RISC-V. In K3s, klipper-helm is used to install Traefik, which is part of the default deployment.

The resulting container image, built with ARCH=riscv64 make has been tested on QEMU and is working fine.