doc: adjust docker related desc in readme

[aroundabout]

Purpose of the PR

• add ASF docker policy
• and recommend users to use the release tag instead of latest

Main Changes

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • xxx

Does this PR potentially affect the following parts?

• Nope
• Dependencies (add/update license info)
• Modify configurations
• The public API
• Other affects (typed here)

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (b066b80) 62.49% compared to head (a22a0d1) 62.47%.
Report is 16 commits behind head on master.

Files	Patch %	Lines
...egraph/loader/direct/loader/HBaseDirectLoader.java	0.00%	2 Missing ⚠️
.../apache/hugegraph/loader/executor/LoadOptions.java	33.33%	2 Missing ⚠️
...e/hugegraph/loader/spark/HugeGraphSparkLoader.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #559      +/-   ##
============================================
- Coverage     62.49%   62.47%   -0.03%     
+ Complexity     1903      930     -973     
============================================
  Files           262       93     -169     
  Lines          9541     4509    -5032     
  Branches        886      529     -357     
============================================
- Hits           5963     2817    -3146     
+ Misses         3190     1483    -1707     
+ Partials        388      209     -179     

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[imbajin]

THX, almost LGTM