Run the cassandra schema job as a non-root user by default

[chgl]

Which problem is this PR solving?

By default, the cassandra schema job runs as a root user. This doesn't work on clusters with restrictive security policies. Although I did create jaegertracing/jaeger-operator#1167 in the past, I think it's more convenient to have this change directly within the schema job and have a more secure default.

Short description of the changes

Adds a new user nonroot to the schema image and sets this as the default user.

[codecov]

Codecov Report

Merging #3043 (e492cb0) into master (394ec23) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3043      +/-   ##
==========================================
- Coverage   96.00%   95.98%   -0.03%     
==========================================
  Files         229      229              
  Lines        9937     9937              
==========================================
- Hits         9540     9538       -2     
- Misses        328      329       +1     
- Partials       69       70       +1     

Impacted Files	Coverage Δ
cmd/query/app/static_handler.go	95.16% <0.00%> (-1.62%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 394ec23...e492cb0. Read the comment docs.