Techdocs without docker-in-docker (#2438)

We're deploying backstage to GKE and have had issues getting techdocs
volume mount logic to work because of mixing paths between the host and
the container running the backend. So we added a condition where if
mkdocs is found in the backend container it will use that to generate
the docs rather than spinning it up in a docker container.

plugins/techdocs-backend/package.json

@@ -25,6 +25,7 @@
     "@backstage/config": "^0.1.1-alpha.21",
     "@types/dockerode": "^2.5.34",
     "@types/express": "^4.17.6",
+    "command-exists-promise": "^2.0.2",
     "default-branch": "^1.0.8",
     "dockerode": "^3.2.1",
     "express": "^4.17.1",