fix: namespace local docker action image tags per repository (#1051)

Fixes #1039 — local docker actions referenced with `uses: ./` are incorrectly served from a cached image built for a *different* repository.

## Root cause

For a local docker action, the built image tag is derived from `actionName`, which is the **workspace-relative** path of the action. For a self-referencing action that is always `"./"`, regardless of which repository it lives in. In `execAsDocker` this collapsed to the constant tag `act-dockeraction:latest` for *every* repository.

Before building, the runner checks `ContainerImageExistsLocally` and skips the rebuild if a matching tag is present. On a shared docker daemon, once repository A built its image, repository B's `uses: ./` found the same tag, skipped its rebuild, and **ran repository A's action image** — the reported "the first action is run again, because the path is cached", including the cross-repository concern raised in the issue.

## Fix

Extracted the tag computation into a small, testable `dockerActionImageTag(repository, actionName, localAction)` function. For local actions the tag is now namespaced with the repository (`path.Join(repository, actionName)`):

- different repositories no longer collide (`act-owner-repo-a-dockeraction:latest` vs `act-owner-repo-b-dockeraction:latest`);
- image caching is preserved within a repository (tag stays stable);
- remote actions are unchanged — they already carry a unique, ref-scoped `actionName` (the uses hash).

---------

Co-authored-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1051
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Co-authored-by: bircni <bircni@icloud.com>
Co-committed-by: bircni <bircni@icloud.com>

act/runner/action_test.go

@@ -455,3 +455,50 @@ func TestExecAsDockerHoldsCloneLockForRemoteUncached(t *testing.T) {
 		t.Fatal("execAsDocker did not return after inner was released and ctx was canceled")
 	}
 }
+
+func TestDockerActionImageTag(t *testing.T) {
+	// Remote actions already carry a unique, ref-scoped actionName (the uses
+	// hash), so the tag must be left untouched for backwards compatibility.
+	assert.Equal(t,
+		"act-abc123-dockeraction:latest",
+		dockerActionImageTag("owner/repo", "abc123", false),
+	)
+
+	// Local actions keep a human-readable, repository-namespaced prefix and gain a short hash suffix that makes the tag unique per (repository, actionName).
+	// See https://gitea.com/gitea/runner/issues/1039.
+	assert.Equal(t,
+		"act-owner-repo-baca2daaa2fe-dockeraction:latest",
+		dockerActionImageTag("owner/repo", "./", true),
+	)
+	assert.Equal(t,
+		"act-owner-repo-sub-e847b61255a8-dockeraction:latest",
+		dockerActionImageTag("owner/repo", "./sub", true),
+	)
+
+	// Sanitizing every non-alphanumeric character to "-" is lossy, so distinct inputs can collapse to the same readable prefix.
+	// The hash suffix must keep such cases apart, otherwise an image built for one repository is reused for another.
+	collisions := [][2]struct {
+		repoName   string
+		actionName string
+	}{
+		// Two different repositories, both `uses: ./`: "a/b-c" and "a-b/c" both sanitize to "a-b-c".
+		{{"a/b-c", "./"}, {"a-b/c", "./"}},
+		// A repository's root action vs another repository's sub-path action:
+		// "owner/repo-a" + "./" and "owner/repo" + "./a" both sanitize to "owner-repo-a".
+		{{"owner/repo-a", "./"}, {"owner/repo", "./a"}},
+	}
+	for _, c := range collisions {
+		assert.NotEqual(t,
+			dockerActionImageTag(c[0].repoName, c[0].actionName, true),
+			dockerActionImageTag(c[1].repoName, c[1].actionName, true),
+			"local docker action tags must differ for %q/%q vs %q/%q",
+			c[0].repoName, c[0].actionName, c[1].repoName, c[1].actionName,
+		)
+	}
+
+	// Distinct local actions within the same repository keep distinct tags.
+	assert.NotEqual(t,
+		dockerActionImageTag("owner/repo", "./", true),
+		dockerActionImageTag("owner/repo", "./sub", true),
+	)
+}