gitssh: Add base image sha to Dockerfile (#21802)

server/gitssh/Dockerfile

@@ -3,7 +3,7 @@
 
 # DisableDockerDetector "No feasible secure solution for OSS repos yet"
 
-FROM alpine:3.12
+FROM alpine:3.12@sha256:c75ac27b49326926b803b9ed43bf088bc220d22556de1bc5f72d742c91398f69
 
 # Install Git and OpenSSH so we can run a git server
 RUN apk add --no-cache git

tools/pipelines/server-gitssh.yml

@@ -90,4 +90,5 @@ extends:
     buildDirectory: server/gitssh
     containerName: fluidframework/routerlicious/gitssh
     setVersion: false
+    enableDockerImagePull: false
     tagName: gitssh