diff --git a/.github/workflows/forward-compatibility.yml b/.github/workflows/forward-compatibility.yml
index 1fab549b5dc3..cbcc1cb07984 100644
--- a/.github/workflows/forward-compatibility.yml
+++ b/.github/workflows/forward-compatibility.yml
@@ -29,27 +29,35 @@ jobs:
     env:
       ARCH: x86_64
       ARCH_CMD: linux64
-      DOCKER_TEST_PREFIX: crystallang/crystal:${{ matrix.crystal_bootstrap_version }}
+      CRYSTAL_BOOTSTRAP_VERSION: ${{ matrix.crystal_bootstrap_version }}
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
         crystal_bootstrap_version: [1.7.3, 1.8.2, 1.9.2, 1.10.1, 1.11.2, 1.12.2, 1.13.3, 1.14.1, 1.15.1, 1.16.3, 1.17.1]
-        flags: [""]
+        env: [{}]
         include:
           # libffi is only available starting from the 1.2.2 build images
+          # pcre2 is only available starting from the 1.7.0 build images
           - crystal_bootstrap_version: 1.1.1
-            flags: "FLAGS=-Dwithout_ffi USE_PCRE1=true"
+            env:
+              FLAGS: -Dwithout_ffi
+              USE_PCRE1: true
           - crystal_bootstrap_version: 1.2.2
-            flags: "USE_PCRE1=true"
+            env:
+              USE_PCRE1: true
           - crystal_bootstrap_version: 1.3.2
-            flags: "USE_PCRE1=true"
+            env:
+              USE_PCRE1: true
           - crystal_bootstrap_version: 1.4.1
-            flags: "USE_PCRE1=true"
+            env:
+              USE_PCRE1: true
           - crystal_bootstrap_version: 1.5.1
-            flags: "USE_PCRE1=true"
+            env:
+              USE_PCRE1: true
           - crystal_bootstrap_version: 1.6.2
-            flags: "USE_PCRE1=true"
+            env:
+              USE_PCRE1: true
     steps:
       - name: Download Crystal source
         uses: actions/checkout@v5
@@ -63,4 +71,5 @@ jobs:
         run: bin/ci prepare_build
 
       - name: Test
-        run: ${{ matrix.flags }} bin/ci build
+        run: bin/ci build
+        env: ${{ matrix.env }}
diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index d30d592b6cd1..a88f5bed050a 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -21,17 +21,18 @@ jobs:
     env:
       ARCH: x86_64
       ARCH_CMD: linux64
-      DOCKER_TEST_PREFIX: crystallang/crystal:${{ matrix.crystal_bootstrap_version }}
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        flags: [""]
+        env: [{}]
         include:
-          # libffi is only available starting from the 1.2.2 build images
-          - crystal_bootstrap_version: 1.0.0
-            flags: "FLAGS=-Dwithout_ffi USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.18.2 # LATEST RELEASE
+          - env:
+              CRYSTAL_BOOTSTRAP_VERSION: 1.0.0
+              # libffi is only available starting from the 1.2.2 build images
+              FLAGS: "-Dwithout_ffi"
+              # pcre2 is only available starting from the 1.7.0 build images
+              USE_PCRE1: true
     steps:
       - name: Download Crystal source
         uses: actions/checkout@v5
@@ -45,7 +46,8 @@ jobs:
         run: bin/ci prepare_build
 
       - name: Test
-        run: ${{ matrix.flags }} bin/ci build
+        run: bin/ci build
+        env: ${{ matrix.env }}
 
   x86_64-musl-test:
     env:
diff --git a/bin/ci b/bin/ci
index fabeae18cbab..a47600b5c952 100755
--- a/bin/ci
+++ b/bin/ci
@@ -189,7 +189,7 @@ with_build_env() {
 
   on_linux verify_linux_environment
 
-  export DOCKER_TEST_PREFIX="${DOCKER_TEST_PREFIX:=crystallang/crystal:1.18.2}"
+  export DOCKER_TEST_PREFIX="${DOCKER_TEST_PREFIX:="crystallang/crystal:${CRYSTAL_BOOTSTRAP_VERSION:-1.18.2}"}"
 
   case $ARCH in
     x86_64)