Relax Dockerfile syntax version specifier

Now that Dockerfile 1.4 is in stable release, this commit will relax the
Dockerfile version from 1.4 to 1, to allow further 1.x releases.

Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.4
+# syntax=docker/dockerfile:1
 ARG PYTHON_VERSION=3.10 LINUX_VERSION=
 FROM python:${PYTHON_VERSION}${LINUX_VERSION:+-$LINUX_VERSION} AS builder
 LABEL org.opencontainers.image.authors="Brendon Smith <[email protected]>"