github: update bug report template (#6076)

To include Docker image SHA

.github/ISSUE_TEMPLATE/bug_report.md

@@ -23,7 +23,7 @@ The issue will be closed without any explanation if it does not satisfy any of f
 **Describe the bug**
 <!-- A clear and concise description of what the bug is -->
 
-**Gogs version or commit**
+**Gogs version, commit or Docker image SHA**
 <!-- The version number or the commit SHA of the Gogs instance you use -->
 
 **Git version**