Automate bumping of Dockerfile syntax line

[pkwarren]

We've now got a Dockerfile tracking docker/dockerfile after landing #1256. We should use this to automate bumping the syntax=docker/dockerfile lines now in the fetcher automatically (this is done manually today). For example: https://github.com/bufbuild/plugins/blob/a5d5af820a04a6bcbcc96c5a2236035e570af2af/plugins/bufbuild/es/v1.9.0/Dockerfile#L1