minor #10490 Updated the command to run the Docker image (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #10490).

Discussion
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Updated the command to run the Docker image

We merged #10131 but the command shown to serve the image locally doesn't work for me. I'm using Docker 18.06.0-ce. The change I made in this PR worked for me ... but we'd need the help of Docker experts to verify and discuss about the best possible command that will work under any circumstances. Thanks!

Commits
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f277bdf Updated the command to run the Docker image

Author: javiereguiluz

README.markdown

@@ -29,6 +29,7 @@ You can build the doc locally with these commands:
 # build the image...
 $ docker build . -t symfony-docs
 
-# ...and serve it locally on http//:127.0.0.1
-$ docker run --rm -p80:80 symfony-docs
+# ...and serve it locally on http//:127.0.0.1:8080
+# (if it's already in use, change the '8080' port by any other port)
+$ docker run --rm -p 8080:80 symfony-docs
 ```