appimage build: pin glibc version in docker image, for reproducibility
appimage build: pin glibc version in docker image, for reproducibility

fixes #6357

wakiyamap/electrum-mona,spesmilo/electrum,wakiyamap/electrum-mona,pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,wakiyamap/electrum-mona,pooler/electrum-ltc,wakiyamap/electrum-mona,pooler/electrum-ltc