add docker badge to readme, fix links for badges

Author: behackl

README.md

@@ -2,11 +2,12 @@
     <img src="https://raw.githubusercontent.com/ManimCommunity/manim/master/logo/cropped.png" />
     <br />
     <br />
-    <img src="https://img.shields.io/badge/license-MIT-red.svg?style=flat" alt="MIT License" src="http://choosealicense.com/licenses/mit/" />
-    <img src="https://img.shields.io/reddit/subreddit-subscribers/manim.svg?color=orange&label=reddit" alt="Reddit" href="https://www.reddit.com/r/manim/" />
-    <img src="https://img.shields.io/discord/581738731934056449.svg?label=discord&color=yellow" alt="Discord" href="https://discord.gg/mMRrZQW" />
-    <img src="https://readthedocs.org/projects/manimce/badge/?version=latest" alt="Documentation Status" href="https://manimce.readthedocs.io/en/latest/?badge=latest" />
-    <img src="https://github.com/ManimCommunity/manim/workflows/CI/badge.svg" alt="CI" />
+    <a href="http://choosealicense.com/licenses/mit/"><img src="https://img.shields.io/badge/license-MIT-red.svg?style=flat" alt="MIT License"></a>
+    <a href="https://www.reddit.com/r/manim/"><img src="https://img.shields.io/reddit/subreddit-subscribers/manim.svg?color=orange&label=reddit" alt="Reddit" href=></a>
+    <a href="https://discord.gg/mMRrZQW"><img src="https://img.shields.io/discord/581738731934056449.svg?label=discord&color=yellow" alt="Discord"></a>
+    <a href="https://manimce.readthedocs.io/en/latest/"><img src="https://readthedocs.org/projects/manimce/badge/?version=latest" alt="Documentation Status"></a>
+    <a href="https://hub.docker.com/r/manimcommunity/manim"><img src="https://img.shields.io/docker/v/manimcommunity/manim?color=%23099cec&label=Docker%20image" alt="Docker image"> </a>
+    <img src="https://github.com/ManimCommunity/manim/workflows/CI/badge.svg" alt="CI">
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     <i>An animation engine for explanatory math videos</i>