Update base image to Alpine 3.12.5
Update base image to Alpine 3.12.5

Alpine 3.12.4 uses a vulnerable version of libssl1.1
(CVE-2021-3449 and CVE-2021-3450), issues that are fixed in Alpine
3.12.5. This is not really a problem for Webdis since it doesn't use
SSL, but the vulnerability shows up on image scans and users who build
images with Webdis as the base image could be at risk if their own
changes depend on this library.

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