docker: install riscv32 toolchain

For building riscv32 targets we should use the riscv32 toolchain.
Add it to the Docker image.

Drop the riscv toolchain-alias as we do not need it in future.

While in here, update to the latest "jammy" tag.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Rick Chen <rick@andestech.com>
[trini: Update to latest jammy tag]
Signed-off-by: Tom Rini <trini@konsulko.com>