unix/linux: adjust Dockerfile ENV key value format

This fixes the following warnings emitted when running mkall.sh on
GOOS=linux with a recent Docker version:

 6 warnings found (use --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 4)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 24)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 25)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 26)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 33)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 56)

Also see https://docs.docker.com/reference/build-checks/legacy-key-value-format/

Change-Id: Icdf05cbe7b8650fc2fec84e010c7401807f5f75f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/621956
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

unix/linux/Dockerfile

@@ -1,7 +1,7 @@
 FROM ubuntu:24.10
 
 # Disable interactive prompts on package installation
-ENV DEBIAN_FRONTEND noninteractive
+ENV DEBIAN_FRONTEND=noninteractive
 
 # Dependencies to get the git sources and go binaries
 RUN apt-get update && apt-get install -y  --no-install-recommends \
@@ -21,16 +21,16 @@ RUN git clone --branch v6.11 --depth 1 https://kernel.googlesource.com/pub/scm/l
 RUN git clone --branch release/2.40/master --depth 1 https://sourceware.org/git/glibc.git
 
 # Get Go
-ENV GOLANG_VERSION 1.23.0
-ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 905a297f19ead44780548933e0ff1a1b86e8327bb459e92f9c0012569f76f5e3
+ENV GOLANG_VERSION=1.23.0
+ENV GOLANG_DOWNLOAD_URL=https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
+ENV GOLANG_DOWNLOAD_SHA256=905a297f19ead44780548933e0ff1a1b86e8327bb459e92f9c0012569f76f5e3
 
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
     && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \
     && tar -C /usr/local -xzf golang.tar.gz \
     && rm golang.tar.gz
 
-ENV PATH /usr/local/go/bin:$PATH
+ENV PATH=/usr/local/go/bin:$PATH
 
 # Linux and Glibc build dependencies and emulator
 RUN apt-get update && apt-get install -y  --no-install-recommends \
@@ -53,6 +53,6 @@ RUN apt-get update && apt-get install -y \
     && rm -rf /var/lib/apt/lists/*
 
 # Let the scripts know they are in the docker environment
-ENV GOLANG_SYS_BUILD docker
+ENV GOLANG_SYS_BUILD=docker
 WORKDIR /build/unix
 ENTRYPOINT ["go", "run", "linux/mkall.go", "/git/linux", "/git/glibc"]