Add reading environment from Docker image (#771)

* fix: read envvars from image definition

* fix: add test

* fix: sort slices

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

act/container/docker_run_test.go

@@ -0,0 +1,34 @@
+package container
+
+import (
+	"sort"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMergeEnvFromImage(t *testing.T) {
+	inputEnv := []string{
+		"PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/sbin",
+		"GOPATH=/root/go",
+		"GOOS=linux",
+	}
+	imageEnv := []string{
+		"PATH=/root/go/bin",
+		"GOPATH=/tmp",
+		"GOARCH=amd64",
+	}
+
+	merged := mergeEnvFromImage(inputEnv, imageEnv)
+	sort.Strings(merged)
+
+	expected := []string{
+		"PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/sbin:/root/go/bin",
+		"GOPATH=/root/go",
+		"GOOS=linux",
+		"GOARCH=amd64",
+	}
+	sort.Strings(expected)
+
+	assert.Equal(t, expected, merged)
+}