Update ENV style to use non-legacy form (#638)

runners/alpine/Dockerfile

@@ -4,11 +4,11 @@ FROM ghcr.io/pyca/static-nodejs-${NODE_ARCH_RELEASE} AS staticnodejs
 FROM alpine:latest
 
 # Increment this to blow away the docker cache
-ENV CACHE_BUSTER 1
+ENV CACHE_BUSTER=1
 
 # This is needed because otherwise `sys.getfilesystemencoding()` returns
 # "ANSI_X3.4-1968".
-ENV LANG C.UTF-8
+ENV LANG=C.UTF-8
 
 RUN apk add --no-cache git libffi-dev curl \
     python3-dev openssl-dev bash gcc musl-dev tar pkgconfig zstd libucontext-dev musl-obstack-dev make

runners/debian/Dockerfile

@@ -2,11 +2,11 @@ ARG RELEASE
 FROM debian:${RELEASE}
 
 # Increment this to blow away the docker cache
-ENV CACHE_BUSTER 5
+ENV CACHE_BUSTER=5
 
 # This is needed because otherwise `sys.getfilesystemencoding()` returns
 # "ANSI_X3.4-1968".
-ENV LANG C.UTF-8
+ENV LANG=C.UTF-8
 
 RUN apt-get -qq update && apt-get install -qq -y \
     build-essential \

runners/fedora/Dockerfile

@@ -2,7 +2,7 @@ FROM registry.fedoraproject.org/fedora:latest
 
 # This is needed because otherwise `sys.getfilesystemencoding()` returns
 # "ANSI_X3.4-1968".
-ENV LANG C.UTF-8
+ENV LANG=C.UTF-8
 
 RUN dnf install -y gcc redhat-rpm-config libffi-devel python3 \
     python3-devel openssl-devel git findutils which pkg-config zstd && \

runners/ubuntu/Dockerfile

@@ -2,13 +2,13 @@ ARG RELEASE
 FROM ubuntu:${RELEASE}
 
 # Increment this to blow away the docker cache
-ENV CACHE_BUSTER 3
+ENV CACHE_BUSTER=3
 
-ENV DEBIAN_FRONTEND noninteractive
+ENV DEBIAN_FRONTEND=noninteractive
 
 # This is needed because otherwise `sys.getfilesystemencoding()` returns
 # "ANSI_X3.4-1968".
-ENV LANG C.UTF-8
+ENV LANG=C.UTF-8
 
 RUN apt-get -qq update && apt-get install -qq -y \
     build-essential \