Add custom Go base container setting

.dagger/modules/e2e/main.dang

@@ -4,8 +4,17 @@ End-to-end checks for the Go module.
 type E2e {
   let generatedFixturePath: String! = "testdata/go-module-generate"
   let generatedFilePath: String! = generatedFixturePath + "/generated.go"
+  let baseFixturePath: String! = "testdata/go-module-custom-base"
+  let baseGeneratedFilePath: String! = baseFixturePath + "/generated.go"
   let emptyFixturePath: String! = "testdata/go-module-empty"
 
+  let base: Container! {
+    container
+      .from("golang:1.25-alpine")
+      .withEnvVariable("DAGGER_GO_CUSTOM_BASE", "yes")
+      .withNewFile("/custom-go-base", "yes\n")
+  }
+
   let assert(condition: Boolean!, message: String!): Void {
     if (condition == false) {
       raise message
@@ -58,19 +67,71 @@ type E2e {
     go(version: "1.25").module(ws, "fixtures/go-module-with-replace").test(ws)
   }
 
+  """
+  A custom base container must be used for Go helpers, tests, and generate.
+  """
+  pub baseCheck(ws: Workspace!): Void @check {
+    let tool = go(
+      base: base,
+      skipLintFilename: ".dagger-skip-lint-disabled",
+      skipTestFilename: ".dagger-skip-test-disabled",
+      skipGenerateFilename: ".dagger-skip-generate-disabled",
+    )
+    let mod = tool.module(ws, baseFixturePath)
+
+    assert(
+      tool.version == null,
+      "base-only constructor exposed a Go version for a black-box base image",
+    )
+
+    let conflict = try {
+      go(version: "1.25", base: base).version ?? "missing conflict"
+    } catch {
+      err => err.message
+    }
+    assert(
+      conflict.contains("version and base are mutually exclusive"),
+      "constructor did not reject version and base together: " + conflict,
+    )
+
+    let testDirs = mod.testDirectories(ws)
+    assert(
+      containsModulePath(testDirs.{path}, baseFixturePath),
+      "custom base test directory discovery did not include expected path",
+    )
+
+    mod.test(ws)
+
+    let changes = mod.generate(ws)
+    assert(
+      containsPath(changes.addedPaths, baseGeneratedFilePath),
+      "custom base generate did not add expected path: " + baseGeneratedFilePath,
+    )
+    assert(
+      changes.layer.file(baseGeneratedFilePath).contents.contains("GeneratedFromBase"),
+      "custom base generate did not produce expected contents",
+    )
+
+    null
+  }
+
   """
   Per-module introspection fields must expose the source selection intermediates.
   """
   pub moduleIntrospectionCheck(ws: Workspace!): Void @check {
     let mod = go(version: "1.25").module(ws, "testdata/go-module-with-testdata/testdata")
 
+    assert(
+      go().version == "1.26",
+      "default constructor did not expose default Go version",
+    )
     assert(
       mod.path == "testdata/go-module-with-testdata",
       "module lookup did not snap to containing module: " + mod.path,
     )
     assert(
       mod.version == "1.25",
-      "module did not expose inherited Go version: " + mod.version,
+      "module did not expose inherited Go version: " + (mod.version ?? "<null>"),
     )
 
     let direct = go(version: "1.25").module(ws, "testdata/go-module-with-testdata/testdata", findUp: false)

go.dang

@@ -6,9 +6,56 @@ https://golang.org
 """
 type Go {
   """
-  Go version used for test and generate commands.
-  """
-  pub version: String! = "1.26"
+  Go version used for the default Go base image. Null when a custom base is set.
+  """
+  pub version: String
+
+  """
+  Base image used for Go test, generate, and helper containers.
+  """
+  pub base: Container!
+
+  new(
+    """
+    Go version used for the default Go base image.
+    """
+    version: String = null,
+    """
+    Base image used for Go test, generate, and helper containers.
+    Mutually exclusive with version.
+    """
+    base: Container = null,
+    """
+    Extra workspace-root include patterns mounted for each module's Go commands.
+    Use this for fixtures, generator inputs, or other files not covered by the
+    built-in Go patterns.
+    """
+    includeExtraFiles: [String!]! = [],
+    """
+    Marker filename that skips lint checks when found at or above a Go module root.
+    """
+    skipLintFilename: String! = ".dagger-skip-go-lint",
+    """
+    Marker filename that skips tests when found at or above a Go module root.
+    """
+    skipTestFilename: String! = ".dagger-skip-go-test",
+    """
+    Marker filename that skips go generate when found at or above a Go module root.
+    """
+    skipGenerateFilename: String! = ".dagger-skip-go-generate",
+  ) {
+    if (version != null and base != null) {
+      raise "version and base are mutually exclusive"
+    } else {
+      self.version = if (base == null) { version ?? "1.26" } else { null }
+      self.base = base ?? container.from("golang:" + (version ?? "1.26") + "-alpine")
+      self.includeExtraFiles = includeExtraFiles
+      self.skipLintFilename = skipLintFilename
+      self.skipTestFilename = skipTestFilename
+      self.skipGenerateFilename = skipGenerateFilename
+      self
+    }
+  }
 
   """
   Extra workspace-root include patterns mounted for each module's Go commands.
@@ -67,6 +114,7 @@ type Go {
         GoModule(
           path: modPath,
           version: version,
+          baseImage: base,
           includeExtraFiles: includeExtraFiles,
           skipLintFilename: skipLintFilename,
           skipTestFilename: skipTestFilename,
@@ -133,6 +181,7 @@ type Go {
         path.trimPrefix("/")
       },
       version: version,
+      baseImage: base,
       includeExtraFiles: includeExtraFiles,
       skipLintFilename: skipLintFilename,
       skipTestFilename: skipTestFilename,
@@ -218,9 +267,14 @@ type GoModule {
   pub path: String!
 
   """
-  Go version used by this module's test and generate containers.
+  Go version configured on the root Go module. Null when a custom base is set.
+  """
+  pub version: String
+
+  """
+  Base image used by this module's Go containers.
   """
-  pub version: String!
+  let baseImage: Container!
 
   """
   Extra workspace-root include patterns mounted for this module's Go commands.
@@ -268,8 +322,7 @@ type GoModule {
   Base container for the Go include helper.
   """
   let goIncludesHelper(ws: Workspace!): Container! {
-    container
-      .from("golang:" + version + "-alpine")
+    baseImage
       .withoutEntrypoint
       .withWorkdir("/helpers/go-includes")
       .withMountedCache("/go/pkg/mod", cacheVolume("go-mod"))
@@ -380,7 +433,7 @@ type GoModule {
           path: testPath,
           ws: ws,
           modulePath: path,
-          version: version,
+          baseImage: baseImage,
           includeExtraFiles: includeExtraFiles,
           skipTestFilename: skipTestFilename,
           testSkipMarkers: testSkipMarkers,
@@ -472,8 +525,7 @@ type GoModule {
   Base Go container before source files and commands are added.
   """
   pub base: Container! {
-    container
-      .from("golang:" + version + "-alpine")
+    baseImage
       .withoutEntrypoint
       .withWorkdir("/ws")
       .withMountedCache("/go/pkg/mod", cacheVolume("go-mod"))
@@ -599,14 +651,14 @@ type GoDirectory {
   let modulePath: String!
 
   """
-  Go version used by this directory's test container.
+  Extra workspace-root include patterns mounted for this directory's Go tests.
   """
-  let version: String!
+  let includeExtraFiles: [String!]!
 
   """
-  Extra workspace-root include patterns mounted for this directory's Go tests.
+  Base image used by this directory's Go containers.
   """
-  let includeExtraFiles: [String!]!
+  let baseImage: Container!
 
   """
   Marker filename that skips tests when found at or above this directory's Go module root.
@@ -637,8 +689,7 @@ type GoDirectory {
   Base container for the Go include helper.
   """
   let goIncludesHelper: Container! {
-    container
-      .from("golang:" + version + "-alpine")
+    baseImage
       .withoutEntrypoint
       .withWorkdir("/helpers/go-includes")
       .withMountedCache("/go/pkg/mod", cacheVolume("go-mod"))
@@ -709,8 +760,7 @@ type GoDirectory {
   Base Go container before source files and commands are added.
   """
   let base: Container! {
-    container
-      .from("golang:" + version + "-alpine")
+    baseImage
       .withoutEntrypoint
       .withWorkdir("/ws")
       .withMountedCache("/go/pkg/mod", cacheVolume("go-mod"))

testdata/go-module-custom-base/.dagger-skip-go-generate

@@ -0,0 +1 @@
+skip

testdata/go-module-custom-base/.dagger-skip-go-lint

@@ -0,0 +1 @@
+skip

testdata/go-module-custom-base/.dagger-skip-go-test

@@ -0,0 +1 @@
+skip

testdata/go-module-custom-base/cmd/writegenerated/main.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	if got := os.Getenv("DAGGER_GO_CUSTOM_BASE"); got != "yes" {
+		panic(fmt.Sprintf("DAGGER_GO_CUSTOM_BASE = %q, want yes", got))
+	}
+	if _, err := os.Stat("/custom-go-base"); err != nil {
+		panic(err)
+	}
+
+	const contents = `package custombase
+
+const GeneratedFromBase = true
+`
+	if err := os.WriteFile("generated.go", []byte(contents), 0o644); err != nil {
+		panic(err)
+	}
+}

testdata/go-module-custom-base/custom_base_test.go

@@ -0,0 +1,15 @@
+package custombase
+
+import (
+	"os"
+	"testing"
+)
+
+func TestBase(t *testing.T) {
+	if got := os.Getenv("DAGGER_GO_CUSTOM_BASE"); got != "yes" {
+		t.Fatalf("DAGGER_GO_CUSTOM_BASE = %q, want yes", got)
+	}
+	if _, err := os.Stat("/custom-go-base"); err != nil {
+		t.Fatalf("custom base marker file is missing: %v", err)
+	}
+}

testdata/go-module-custom-base/generate.go

@@ -0,0 +1,3 @@
+package custombase
+
+//go:generate go run ./cmd/writegenerated

testdata/go-module-custom-base/go.mod

@@ -0,0 +1,3 @@
+module example.com/custombase
+
+go 1.25