Upgrade alpine to 3.13 (#216)

The docker-vault image uses alpine:3.10 since Jul 9, 2019.
An upgrade is advisable since the packages 'libcrypto' 1.1.1g-r0, 'libssl' 1.1.1g-r0, and 'musl-utils' 1.1.22-r3 have CVE-2020-1971 and CVE-2020-28928 reported.
Aqua trivy and Prisma Cloud twistcli both reports these issues.