Pin the FROM in dockerfiles (#19526)

This prevents supply chain attacks where the latest image is replaced with a malicious version.

Pinning done by using https://github.com/Jille/dockpin

build/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.14
+FROM alpine:3.14@sha256:635f0aa53d99017b38d1a0aa5b2082f7812b03e3cdb299103fe77b5c8a07f1d2
 
 # Some ENV variables
 ENV PATH="/mattermost/bin:${PATH}"