devnet: pin Dockerfile syntax to digest

This speeds up the build by ~1.5s by avoiding a remote lookup on
Docker Hub every time this is built (WTF).

Dockerfile.agent

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:experimental
+# syntax=docker.io/docker/dockerfile:experimental@sha256:de85b2f3a3e8a2f7fe48e8e84a65f6fdd5cd5183afa6412fff9caa6871649c44
 FROM rust:1.45
 
 RUN apt-get update && apt-get install -y libssl-dev libudev-dev pkg-config zlib1g-dev llvm clang ncat

bridge/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:experimental
+# syntax=docker.io/docker/dockerfile:experimental@sha256:de85b2f3a3e8a2f7fe48e8e84a65f6fdd5cd5183afa6412fff9caa6871649c44
 FROM golang:1.15.5
 
 WORKDIR /app

ethereum/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:experimental
+# syntax=docker.io/docker/dockerfile:experimental@sha256:de85b2f3a3e8a2f7fe48e8e84a65f6fdd5cd5183afa6412fff9caa6871649c44
 FROM node:lts-alpine
 
 # npm wants to clone random Git repositories - lovely.

solana/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:experimental
+# syntax=docker.io/docker/dockerfile:experimental@sha256:de85b2f3a3e8a2f7fe48e8e84a65f6fdd5cd5183afa6412fff9caa6871649c44
 FROM rust:1.45
 
 RUN apt-get update && apt-get install -y libssl-dev libudev-dev pkg-config zlib1g-dev llvm clang

third_party/solana/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:experimental
+# syntax=docker.io/docker/dockerfile:experimental@sha256:de85b2f3a3e8a2f7fe48e8e84a65f6fdd5cd5183afa6412fff9caa6871649c44
 
 # Depend on our smart contract build - it's going to be linked directly into Solana as part the patch we carry.
 FROM solana-contract as contract