Development docs warning for beta-docs.docker.io

Update: Fix alignment issues of the "warning" block.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)

Docker-DCO-1.1-Signed-off-by: O.S.Tezer <ostezer@gmail.com> (github: SvenDowideit)

docs/theme/mkdocs/base.html

@@ -48,6 +48,7 @@
             <div class="col-sm-8" role="main">
               {% include "breadcrumbs.html" %}
               <main id="content" role="main">
+                {% include "beta_warning.html" %}
                 {{ content }}
               </main>
             </div>

docs/theme/mkdocs/beta_warning.html

@@ -0,0 +1,27 @@
+<style>
+.bs-callout {
+  padding: 20px;
+  border-left: 3px solid rgb(238, 238, 238);
+  width: 100% !important;
+}
+.bs-callout p {
+  margin-bottom: 0 !important;
+}
+.bs-callout h4 span {
+  font-style: italic;
+  font-weight: bold;
+}
+.bs-callout-danger {
+  background-color: rgb(253, 247, 247);
+  border-color: rgb(217, 83, 79);
+}
+.bs-callout h4 {
+  margin-top: 0;
+  margin-bottom: 5px;
+  color: rgb(217, 83, 79);
+}
+</style>
+<div class="bs-callout bs-callout-danger">
+<h4>You are looking at the <span>beta</span> docs for the development version of Docker.</h4>
+There is a chance of them being different from the prior versions.
+</div>