1. Updates to image which is being actively maintained.

2. Adopts environment variable conventions of new image.

3. Uses custom MariaDB instance as back-end.

4. Removes `/etc/timezone` mapping (without replacing with `TZ`)
   because new image is built without `tzdata`.

5. Adds basic documentation.

Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>

When HTTPS was enabled, the healthcheck script failed for a number of
reasons, not the least of which were `curl` needing to be provided with
the path to the container's self-signed certificate and problems
associated with using "localhost" rather than the container name.

In theory, `gitea cert` will generate for `--host gitea,localhost` and
those do turn up in the certificate. But `curl` doesn't seem to like it.
Rather than try to figure out why `curl` gets upset, it's easier to just
use "hostname" syntax in the healthcheck URL. In other words:

```
https://gitea:3000
```

rather than:

```
https://localhost:3000
```

Although it isn't strictly necessary for HTTP, I used "hostname"
syntax for that URL too, for consistency.

Unlike `localhost`, "hostname" syntax also steers clear of IPv6 `::1`.

Documentation updated to include instructions for swapping the
healthcheck URLs when enabling HTTPS.

Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>