Fix ubuntu-focal Docker build: update NodeSource setup and ENV format

- Replace deprecated apt-key NodeSource setup (node_16.x) with
  modern gpg keyring method using node_20.x/nodistro, fixing GPG
  signature verification failure
- Convert all legacy ENV key value syntax to ENV key=value format
  to eliminate 15 Dockerfile linting warnings

Client: build

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Generated-by: Claude Sonnet 4.6
diff --git a/build/docker/ubuntu-focal/Dockerfile b/build/docker/ubuntu-focal/Dockerfile
index 855ca98..ea1868a 100644
--- a/build/docker/ubuntu-focal/Dockerfile
+++ b/build/docker/ubuntu-focal/Dockerfile
@@ -58,8 +58,9 @@
       chown root:root /etc/apt/sources.list.d/microsoft-prod.list
 
 # node.js
-RUN curl -sL https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add - && \
-      echo "deb https://deb.nodesource.com/node_16.x focal main" | tee /etc/apt/sources.list.d/nodesource.list
+RUN mkdir -p /etc/apt/keyrings && \
+      curl -fsSL https://deb.nodesource.com/gpgkey/nodesource-repo.gpg.key | gpg --dearmor -o /etc/apt/keyrings/nodesource.gpg && \
+      echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_20.x nodistro main" | tee /etc/apt/sources.list.d/nodesource.list
 
 ### install general dependencies
 RUN apt-get update -yq && \
@@ -82,14 +83,14 @@
       unzip \
       valgrind \
       vim
-ENV PATH /usr/lib/llvm-6.0/bin:$PATH
+ENV PATH=/usr/lib/llvm-6.0/bin:$PATH
 
 # lib/as3 (ActionScript)
 RUN mkdir -p /usr/local/adobe/flex/4.6 && \
       cd /usr/local/adobe/flex/4.6 && \
       wget -q "http://download.macromedia.com/pub/flex/sdk/flex_sdk_4.6.zip" && \
       unzip flex_sdk_4.6.zip
-ENV FLEX_HOME /usr/local/adobe/flex/4.6
+ENV FLEX_HOME=/usr/local/adobe/flex/4.6
 
 # TODO: "apt-get install" without "apt-get update" in the same "RUN" step can cause cache issues if modified later.
 # See https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#run
@@ -102,7 +103,7 @@
       qtbase5-dev \
       qtbase5-dev-tools
 
-ENV SBCL_VERSION 1.5.3
+ENV SBCL_VERSION=1.5.3
 RUN \
       `# Common Lisp (sbcl) dependencies` \
       curl --version && \
@@ -114,8 +115,8 @@
       cd .. && \
       rm -rf sbcl*
 
-ENV D_VERSION     2.087.0
-ENV DMD_DEB       dmd_2.087.0-0_amd64.deb
+ENV D_VERSION=2.087.0
+ENV DMD_DEB=dmd_2.087.0-0_amd64.deb
 RUN \
       `# D dependencies` \
       wget -q http://downloads.dlang.org/releases/2.x/${D_VERSION}/${DMD_DEB} && \
@@ -131,11 +132,11 @@
       mv deimos-openssl-1.1.0h/C/* /usr/include/dmd/druntime/import/C/ && \
       rm -rf deimos-openssl-1.1.0h
 
-ENV DART_VERSION 2.7.2-1
+ENV DART_VERSION=2.7.2-1
 RUN apt-get install -y --no-install-recommends \
       `# Dart dependencies` \
       dart=$DART_VERSION
-ENV PATH /usr/lib/dart/bin:$PATH
+ENV PATH=/usr/lib/dart/bin:$PATH
 
 # Because Ubuntu 20.04 turned EOL in April/May 2025, Microsoft does not support .NET 9 or higher on that platform
 RUN apt-get install -y --no-install-recommends \
@@ -153,16 +154,16 @@
       kerl build $ERLANG_OTP_VERSION && kerl install $ERLANG_OTP_VERSION /usr/local/lib/otp/ && . /usr/local/lib/otp/activate && \
       curl -ssLo /usr/local/bin/rebar3 https://github.com/erlang/rebar3/releases/download/${ERLANG_REBAR_VERSION}/rebar3 && chmod +x /usr/local/bin/rebar3 && \
       rebar3 --version
-ENV PATH /usr/local/lib/otp/bin:$PATH
+ENV PATH=/usr/local/lib/otp/bin:$PATH
 
 RUN apt-get install -y --no-install-recommends \
       `# GlibC dependencies` \
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.24.3
-ENV GOLANG_DOWNLOAD_URL https://go.dev/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 3333f6ea53afa971e9078895eaa4ac7204a8c6b5c68c10e6bc9a33e8e391bdd8
+ENV GOLANG_VERSION=1.24.3
+ENV GOLANG_DOWNLOAD_URL=https://go.dev/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
+ENV GOLANG_DOWNLOAD_SHA256=3333f6ea53afa971e9078895eaa4ac7204a8c6b5c68c10e6bc9a33e8e391bdd8
 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 && \
@@ -182,7 +183,7 @@
     tar xvf neko-${NEKO_VERSION}-linux64.tar.gz && \
     rm neko-${NEKO_VERSION}-linux64.tar.gz && \
     mv neko-* /opt/neko
-ENV PATH /opt/haxe:/opt/neko:$PATH
+ENV PATH=/opt/haxe:/opt/neko:$PATH
 RUN echo "/opt/neko" > /etc/ld.so.conf.d/neko.conf && \
     ldconfig
 USER ${user}
@@ -277,7 +278,7 @@
 USER ${user}
 RUN `# Rust dependencies` \
     curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.83.0 -y
-ENV PATH /home/${user}/.cargo/bin:$PATH
+ENV PATH=/home/${user}/.cargo/bin:$PATH
 USER root
 
 # Swift on Linux for cross tests
@@ -292,7 +293,7 @@
       mv swift-5.7-RELEASE-ubuntu20.04 /usr/share/swift && \
       rm swift-5.7-RELEASE-ubuntu20.04.tar.gz
 
-ENV PATH /usr/share/swift/usr/bin:$PATH
+ENV PATH=/usr/share/swift/usr/bin:$PATH
 RUN swift --version
 
 # Locale(s) for cpp unit tests
@@ -316,7 +317,7 @@
 #     rm -rf /tmp/* && \
 #     rm -rf /var/tmp/*
 
-ENV THRIFT_ROOT /thrift
+ENV THRIFT_ROOT=/thrift
 RUN mkdir -p $THRIFT_ROOT/src && \
     chown -R ${uid}:${uid} $THRIFT_ROOT/
 COPY Dockerfile $THRIFT_ROOT/