diff --git a/ci/alpine/Dockerfile b/ci/alpine/Dockerfile
index bbc9f6ed..ac40431d 100644
--- a/ci/alpine/Dockerfile
+++ b/ci/alpine/Dockerfile
@@ -2,7 +2,7 @@ FROM alpine:latest
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230823
+ENV DOCKERFILE_VERSION=20230823
 
 RUN apk add --no-cache \
   bash \
diff --git a/ci/centos-stream-9/Dockerfile b/ci/centos-stream-9/Dockerfile
index 2243f487..64953c88 100644
--- a/ci/centos-stream-9/Dockerfile
+++ b/ci/centos-stream-9/Dockerfile
@@ -2,7 +2,7 @@ FROM quay.io/centos/centos:stream9
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20220519
+ENV DOCKERFILE_VERSION=20220519
 
 RUN dnf -y install \
     cmake \
diff --git a/ci/debian-11/Dockerfile b/ci/debian-11/Dockerfile
index a94600c5..658fb071 100644
--- a/ci/debian-11/Dockerfile
+++ b/ci/debian-11/Dockerfile
@@ -4,7 +4,7 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230813
+ENV DOCKERFILE_VERSION=20230813
 
 RUN apt-get update && apt-get -y install \
     cmake \
diff --git a/ci/debian-12/Dockerfile b/ci/debian-12/Dockerfile
index 4dfc0bc6..997aefe2 100644
--- a/ci/debian-12/Dockerfile
+++ b/ci/debian-12/Dockerfile
@@ -4,7 +4,7 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230612
+ENV DOCKERFILE_VERSION=20230612
 
 RUN apt-get update && apt-get -y install \
     clang \
diff --git a/ci/fedora-35/Dockerfile b/ci/fedora-35/Dockerfile
index 6e39d91c..dd216111 100644
--- a/ci/fedora-35/Dockerfile
+++ b/ci/fedora-35/Dockerfile
@@ -2,7 +2,7 @@ FROM fedora:35
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20220519
+ENV DOCKERFILE_VERSION=20220519
 
 RUN dnf -y install \
     cmake \
diff --git a/ci/fedora-40/Dockerfile b/ci/fedora-40/Dockerfile
index af5bd0fc..7c615c1d 100644
--- a/ci/fedora-40/Dockerfile
+++ b/ci/fedora-40/Dockerfile
@@ -2,7 +2,7 @@ FROM fedora:40
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20241106
+ENV DOCKERFILE_VERSION=20241106
 
 RUN dnf -y install \
     cmake \
diff --git a/ci/fedora-41/Dockerfile b/ci/fedora-41/Dockerfile
index b29a73f4..2adcb4f9 100644
--- a/ci/fedora-41/Dockerfile
+++ b/ci/fedora-41/Dockerfile
@@ -2,7 +2,7 @@ FROM fedora:41
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20241204
+ENV DOCKERFILE_VERSION=20241204
 
 RUN dnf -y install \
     cmake \
diff --git a/ci/opensuse-tumbleweed/Dockerfile b/ci/opensuse-tumbleweed/Dockerfile
index cbc04e77..02fd2500 100644
--- a/ci/opensuse-tumbleweed/Dockerfile
+++ b/ci/opensuse-tumbleweed/Dockerfile
@@ -2,7 +2,7 @@ FROM opensuse/tumbleweed
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230329
+ENV DOCKERFILE_VERSION=20230329
 
 RUN zypper in -y \
     cmake \
diff --git a/ci/ubuntu-18.04/Dockerfile b/ci/ubuntu-18.04/Dockerfile
index 670597a1..c98951a3 100644
--- a/ci/ubuntu-18.04/Dockerfile
+++ b/ci/ubuntu-18.04/Dockerfile
@@ -4,7 +4,7 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20220519
+ENV DOCKERFILE_VERSION=20220519
 
 ENV CMAKE_DIR "/opt/cmake"
 ENV CMAKE_VERSION "3.19.1"
diff --git a/ci/ubuntu-22.04/Dockerfile b/ci/ubuntu-22.04/Dockerfile
index 38ffc1b3..538608ca 100644
--- a/ci/ubuntu-22.04/Dockerfile
+++ b/ci/ubuntu-22.04/Dockerfile
@@ -4,7 +4,7 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230813
+ENV DOCKERFILE_VERSION=20230813
 
 RUN apt-get update && apt-get -y install \
     cmake \
diff --git a/ci/ubuntu-24.04/Dockerfile b/ci/ubuntu-24.04/Dockerfile
index 00dc4b33..bad36013 100644
--- a/ci/ubuntu-24.04/Dockerfile
+++ b/ci/ubuntu-24.04/Dockerfile
@@ -4,7 +4,7 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20240510
+ENV DOCKERFILE_VERSION=20240510
 
 RUN apt-get update && apt-get -y install \
     cmake \
diff --git a/ci/ubuntu-24.10/Dockerfile b/ci/ubuntu-24.10/Dockerfile
index 252a1b28..4c111f40 100644
--- a/ci/ubuntu-24.10/Dockerfile
+++ b/ci/ubuntu-24.10/Dockerfile
@@ -4,7 +4,7 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20241204
+ENV DOCKERFILE_VERSION=20241204
 
 RUN apt-get update && apt-get -y install \
     cmake \
diff --git a/ci/windows/Dockerfile b/ci/windows/Dockerfile
index 6832104f..7d97dc03 100755
--- a/ci/windows/Dockerfile
+++ b/ci/windows/Dockerfile
@@ -18,7 +18,7 @@ FROM mcr.microsoft.com/dotnet/framework/sdk:4.8-windowsservercore-ltsc2019
 
 # A version field to invalidate Cirrus's build cache when needed, as suggested in
 # https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230728
+ENV DOCKERFILE_VERSION=20230728
 
 SHELL [ "powershell" ]