Fix environment variable replacement in image configuration

image.go

@@ -3,6 +3,7 @@ package dalec
 import (
 	"context"
 	goerrors "errors"
+	"strings"
 
 	"github.com/google/shlex"
 	"github.com/moby/buildkit/client/llb"
@@ -105,11 +106,24 @@ func MergeImageConfig(dst *DockerImageConfig, src *ImageConfig) error {
 		// If the env var already exists, replace it
 		envIdx := make(map[string]int)
 		for i, env := range dst.Env {
-			envIdx[env] = i
+			// Extract the environment variable name (part before '=')
+			varName, _, found := strings.Cut(env, "=")
+			if found {
+				envIdx[varName] = i
+			} else {
+				// Environment variable without '=' - use the whole string as key
+				envIdx[env] = i
+			}
 		}
 
 		for _, env := range src.Env {
-			if idx, ok := envIdx[env]; ok {
+			// Extract the environment variable name from the new env var
+			varName, _, found := strings.Cut(env, "=")
+			if !found {
+				varName = env
+			}
+
+			if idx, ok := envIdx[varName]; ok {
 				dst.Env[idx] = env
 			} else {
 				dst.Env = append(dst.Env, env)

image_env_test.go

@@ -0,0 +1,90 @@
+package dalec
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+	"gotest.tools/v3/assert/cmp"
+)
+
+func TestMergeImageConfigEnvReplacement(t *testing.T) {
+	t.Run("env vars from image config should replace base image env vars", func(t *testing.T) {
+		// Setup: base image with PATH environment variable
+		dst := &DockerImageConfig{}
+		dst.Env = []string{"PATH=/usr/bin:/bin", "HOME=/root"}
+
+		// Image config wants to override PATH
+		src := &ImageConfig{
+			Env: []string{"PATH=/custom/bin:/custom/usr/bin"},
+		}
+
+		err := MergeImageConfig(dst, src)
+		assert.NilError(t, err)
+
+		// PATH should be replaced, not duplicated
+		assert.Check(t, cmp.Len(dst.Env, 2))
+		assert.Check(t, cmp.Contains(dst.Env, "PATH=/custom/bin:/custom/usr/bin"))
+		assert.Check(t, cmp.Contains(dst.Env, "HOME=/root"))
+
+		// Should NOT have duplicate PATH entries
+		pathCount := 0
+		for _, env := range dst.Env {
+			if len(env) >= 5 && env[:5] == "PATH=" {
+				pathCount++
+			}
+		}
+		assert.Check(t, cmp.Equal(pathCount, 1), "Expected exactly 1 PATH env var, got %d", pathCount)
+	})
+
+	t.Run("env vars with different names should be appended", func(t *testing.T) {
+		dst := &DockerImageConfig{}
+		dst.Env = []string{"PATH=/usr/bin:/bin", "HOME=/root"}
+
+		src := &ImageConfig{
+			Env: []string{"USER=myuser", "LANG=en_US.UTF-8"},
+		}
+
+		err := MergeImageConfig(dst, src)
+		assert.NilError(t, err)
+
+		assert.Check(t, cmp.Len(dst.Env, 4))
+		assert.Check(t, cmp.Contains(dst.Env, "PATH=/usr/bin:/bin"))
+		assert.Check(t, cmp.Contains(dst.Env, "HOME=/root"))
+		assert.Check(t, cmp.Contains(dst.Env, "USER=myuser"))
+		assert.Check(t, cmp.Contains(dst.Env, "LANG=en_US.UTF-8"))
+	})
+
+	t.Run("multiple env var replacements", func(t *testing.T) {
+		dst := &DockerImageConfig{}
+		dst.Env = []string{"PATH=/usr/bin:/bin", "HOME=/root", "USER=root"}
+
+		src := &ImageConfig{
+			Env: []string{"PATH=/custom/bin", "USER=customuser"},
+		}
+
+		err := MergeImageConfig(dst, src)
+		assert.NilError(t, err)
+
+		assert.Check(t, cmp.Len(dst.Env, 3))
+		assert.Check(t, cmp.Contains(dst.Env, "PATH=/custom/bin"))
+		assert.Check(t, cmp.Contains(dst.Env, "HOME=/root"))
+		assert.Check(t, cmp.Contains(dst.Env, "USER=customuser"))
+	})
+
+	t.Run("env var without equals sign should not cause issues", func(t *testing.T) {
+		dst := &DockerImageConfig{}
+		dst.Env = []string{"PATH=/usr/bin:/bin", "VALID_VAR"}
+
+		src := &ImageConfig{
+			Env: []string{"PATH=/custom/bin", "ANOTHER_VALID_VAR"},
+		}
+
+		err := MergeImageConfig(dst, src)
+		assert.NilError(t, err)
+
+		assert.Check(t, cmp.Len(dst.Env, 3))
+		assert.Check(t, cmp.Contains(dst.Env, "PATH=/custom/bin"))
+		assert.Check(t, cmp.Contains(dst.Env, "VALID_VAR"))
+		assert.Check(t, cmp.Contains(dst.Env, "ANOTHER_VALID_VAR"))
+	})
+}