backend: Rename build variable DOCKER_COMMIT_HASH -> MEMPOOL_COMMIT_HASH

This var is useful for all build methods, not only Docker.

This is an internal renaming that doesn't change the public Docker
backend image API.

docker/backend/Dockerfile

@@ -1,7 +1,7 @@
 FROM node:16.16.0-buster-slim AS builder
 
 ARG commitHash
-ENV DOCKER_COMMIT_HASH=${commitHash}
+ENV MEMPOOL_COMMIT_HASH=${commitHash}
 
 WORKDIR /build
 COPY . .