Add Health Check endpoint

[mckennajones]

Adds a simple health check endpoint as mentioned in #375. The endpoint at /health simply returns the version of the app and a health status. It will return 'healthy` unless it is unable to connect to the DB. Could easily add other checks as deemed appropriate.

Also added the health check to the Dockerfile so it will report it's health status. Example:

(venv) ➜  ~/temp/linkding git:(feature/health-endpoint) docker-compose up -d
[+] Running 1/1
 ⠿ Container linkding  Started                                                                                                                                         0.3s
(venv) ➜  ~/temp/linkding git:(feature/health-endpoint) docker ps
CONTAINER ID   IMAGE             COMMAND            CREATED          STATUS                            PORTS                    NAMES
19cd852b2d49   linkding-test:1   "./bootstrap.sh"   39 seconds ago   Up 3 seconds (health: starting)   0.0.0.0:9090->9090/tcp   linkding
(venv) ➜  ~/temp/linkding git:(feature/health-endpoint) docker ps
CONTAINER ID   IMAGE             COMMAND            CREATED          STATUS                             PORTS                    NAMES
19cd852b2d49   linkding-test:1   "./bootstrap.sh"   51 seconds ago   Up 15 seconds (health: starting)   0.0.0.0:9090->9090/tcp   linkding
(venv) ➜  ~/temp/linkding git:(feature/health-endpoint) docker ps
CONTAINER ID   IMAGE             COMMAND            CREATED              STATUS                    PORTS                    NAMES
19cd852b2d49   linkding-test:1   "./bootstrap.sh"   About a minute ago   Up 38 seconds (healthy)   0.0.0.0:9090->9090/tcp   linkding
(venv) ➜  ~/temp/linkding git:(feature/health-endpoint) rm -rf ./data

Wait 1 min 30s for 3 retries to occur.

(venv) ➜  ~/temp/linkding git:(feature/health-endpoint) docker ps
CONTAINER ID   IMAGE             COMMAND            CREATED         STATUS                     PORTS                    NAMES
19cd852b2d49   linkding-test:1   "./bootstrap.sh"   2 minutes ago   Up 2 minutes (unhealthy)   0.0.0.0:9090->9090/tcp   linkding

Btw, I'm not super familiar with django. Let me know if a middleware is not the appropriate way to add an endpoint like this.

Closes #375

[mckennajones]

Thanks for the commits @sissbruecker 🙂. Should be closer now.

I also saw this older issue #272 and PR #273 that might be able to be closed out with the addition of this endpoint. Unless there is a use-case for an endpoint that solely returns the version.

[sissbruecker]

Thanks, looks good now 👍

Thanks for the heads up, yes I think the health check endpoint should suffice for retrieving the version for now.