Improve installation instructions

Author: Anze

.env.example

@@ -1,3 +1,3 @@
 BACKEND_URL=https://grafolean.com/api
 BOT_TOKEN=
-JOBS_REFRESH_INTERVAL=60
+JOBS_REFRESH_INTERVAL=60

README.md

@@ -30,11 +30,17 @@ Docker is the easiest and currently the only officially supported way. Note that
     $ wget https://gitlab.com/grafolean/grafolean-collector-ping/raw/master/docker-compose.yml
     ```
 
-3) edit `docker-compose.yml` and change:
+3) These settings must be set:
+
     - mandatory: `BACKEND_URL` (set to the URL of Grafolean backend, for example `https://grafolean.com/api`),
     - mandatory: `BOT_TOKEN` (set to the bot token from step 1),
     - optional: `JOBS_REFRESH_INTERVAL` (interval in seconds at which the jobs definitions will be updated)
-   Alternatively, you can also copy `.env.example` to `.env` and change settings there (leaving `docker-compose.yml` in original state).
+
+   The easiest way to set them is to download example `.env` and edit it:
+    ```
+    $ wget https://gitlab.com/grafolean/grafolean-collector-ping/raw/master/.env.example -O .env
+    $ nano .env
+    ```
 
 4) run: `docker-compose up -d`
 

docker-compose.yml

@@ -10,9 +10,6 @@ services:
     #   image: grafolean/grafolean-collector-ping:v1.0.0
     image: grafolean/grafolean-collector-ping
     container_name: grafolean-collector-ping
-    build:
-      context: .
-      dockerfile: Dockerfile
     environment:
       # Backend url must be set to the address of the Grafolean backend, for example this uses Grafolean hosted service:
       # - BACKEND_URL=https://grafolean.com/api