Update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: "[BUG]"
+labels: ''
+assignees: guzongmin
+
+---
+
+<!--- Provide a general summary of the issue in the Title above -->
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. 
+2. 
+3. 
+4. 
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Logs**
+If applicable, add logs to help explain your problem.
+
+**Environment (please complete the following information):**
+- HW: [e.g. SGX1, SGX2] 
+- OS: [e.g. Ubuntu18.04, CentOS8.1]
+ - Occlum version [e.g. 0.16.0]
+
+**Additional context**
+Add any other context about the problem here.
+
+**Possible Solution/Implementation**
+Not obligatory, but suggest a fix/reason for the bug.