Fix Darion directory traversal vuln

[sargun]

This uses securejoin to ensure that the path of logs are underneath
a the container's log root directory.

[coveralls]

Pull Request Test Coverage Report for Build 3198

• 4 of 8 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 25.971%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
darion/api/handlers.go	4	8	50.0%

Totals
Change from base Build 3146:	0.01%
Covered Lines:	3003
Relevant Lines:	11563

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #231 into master will decrease coverage by <.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
- Coverage   35.75%   35.74%   -0.01%     
==========================================
  Files          71       71              
  Lines        9046     9052       +6     
==========================================
+ Hits         3234     3236       +2     
- Misses       5415     5418       +3     
- Partials      397      398       +1

Impacted Files	Coverage Δ
darion/api/handlers.go	64.23% <42.85%> (-1.28%)	⬇️