docker: build QEMU v8.2.0

ACPI support for RISC-V requires a recent QEMU.

Upgrade the used QEMU to v8.2.0.

QEMU commit 0c7ffc977195 ("hw/net: cadence_gem: Fix MDIO_OP_xxx values")
is needed to fix the Ethernet PHY driver used by the emulated SiFive
Unleashed Board emulation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>