Fix some minor errors

Docker.md

@@ -93,6 +93,7 @@ To install docker, you can simply run:
 ```bash
 $ sudo apt-get install docker
 $ sudo systemctl --now enable dockerd
+$ sudo usermod -a -G docker myuser
 ```
 
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@@ -213,8 +214,10 @@ Docker define a DSL (Domain Specific Language) to declare each of this steps.
 ## Containers DIY: An example
 
 ```Dockerfile
-FROM alpine
-RUN echo "Hello World!"
+FROM alpine:latest
+RUN apk add curl
+
+ENTRYPOINT ["curl"]
 ```
 
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