Dockerfile: fix typo in build vs. target platform

Signed-off-by: John McBride <[email protected]>

Author: jpmcb

Dockerfile

@@ -12,9 +12,9 @@
 # remote code execution vulnerabilities.
 #
 # Uses the bullseye-slim debian image per the rust recommendation.
-FROM --platform=$BUILDPLATFORM rust:1.71-slim-bullseye AS builder
+FROM --platform=$TARGETPLATFORM rust:1.71-slim-bullseye AS builder
 
-ARG BUILDPLATFORM
+ARG TARGETPLATFORM
 
 # Install g++ and other build essentials for compiling openssl/tls dependencies
 RUN apt update