dokku: get buildpack deployment back

Dokku normally defaults to using heroku buildpacks for deployment, but
this may be overridden by committing a valid Dockerfile to the root of
the repository and pushing the repository to the Dokku installation.

Commit 161e556 ("docker: add `Dockerfile` to build Docker images") just
added such a Dockerfile and accidentally switched the deployment to the
`Dockerfile` mode. Due to this fact, Dokku `apt` plugin on the server
started to ignore the ./apt-packages file with a needed dependency for
the app as it works with buildpack-based applications only. So we had
a wrong deployment on the server.

The solution is simple, just move ./Dockerfile to the ./docker dir.

Also, move the ./test dir to ./docker for consistency.

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Build docker image
-        run: docker build --tag rws --no-cache .
+        run: docker build --tag rws --no-cache --file docker/Dockerfile .
 
       - name: Run app in docker container
         run: |

README.md

@@ -193,7 +193,7 @@ python3 -c "from werkzeug.security import generate_password_hash; print(generate
 For running RWS via Docker, just create an image and run a container from it.
 
 ```bash
-docker build -t rws .
+docker build -t rws -f docker/Dockerfile .
 docker run \
     -e RWS_CFG=/app/config.default \
     -e RWS_CREDENTIALS='{"user": "password"}' \
@@ -206,12 +206,14 @@ docker run \
     rws
 ```
 
-## Test stand
+### Test stand
+
+Note, that a Docker image with tag `rws` must be build first.
 
 For setting up a test stand, `docker-compose` can be used:
 
 ```bash
-docker-compose -f test/docker-compose.yml up
+docker-compose -f docker/test/docker-compose.yml up
 ```
 
 It will run RWS and MinIO (S3 storage) in the separate Docker containers.