Lock TechDocs generation to specific container version.

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

.changeset/techdocs-sad-songs-waltzes.md

@@ -0,0 +1,6 @@
+---
+'@backstage/techdocs-common': patch
+---
+
+Locks the version of the default docker image used to generate TechDocs. As of
+this changelog entry, it is v0.3.2!

packages/techdocs-common/api-report.md

@@ -256,6 +256,7 @@ export class TechdocsGenerator implements GeneratorBase {
     config: Config;
     scmIntegrations: ScmIntegrationRegistry;
   });
+  static readonly defaultDockerImage = 'spotify/techdocs:v0.3.2';
   // (undocumented)
   static fromConfig(
     config: Config,

packages/techdocs-common/src/stages/generate/techdocs.ts

@@ -37,9 +37,11 @@ import {
   GeneratorRunOptions,
 } from './types';
 
-const defaultDockerImage = 'spotify/techdocs';
-
 export class TechdocsGenerator implements GeneratorBase {
+  /**
+   * The default docker image used to generate cnotent.
+   */
+  public static readonly defaultDockerImage = 'spotify/techdocs:v0.3.2';
   private readonly logger: Logger;
   private readonly containerRunner: ContainerRunner;
   private readonly options: GeneratorConfig;
@@ -124,7 +126,8 @@ export class TechdocsGenerator implements GeneratorBase {
           break;
         case 'docker':
           await this.containerRunner.runContainer({
-            imageName: this.options.dockerImage ?? defaultDockerImage,
+            imageName:
+              this.options.dockerImage ?? TechdocsGenerator.defaultDockerImage,
             args: ['build', '-d', '/output'],
             logStream,
             mountDirs,