Don't crash if encoded Docker password contains +

[remram44]

This is a different bug from #2167. The decoding is done using URL encoding (A-Za-z-_) instead of standard encoding (A-Za-z+/).

This would cause a crash if a Docker password happened to encoding to a string with a + or /, as was my case with my quay.io account:

$ containerlab deploy
INFO[0000] Containerlab v0.57.0 started                 
INFO[0000] Parsing & checking topology file: frr.clab.yml 
Error: illegal base64 data at input byte 15

[hellt]

appreciate!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.28%. Comparing base (9193652) to head (5c10822).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2187      +/-   ##
==========================================
- Coverage   51.32%   51.28%   -0.04%     
==========================================
  Files         171      171              
  Lines       12537    12537              
==========================================
- Hits         6435     6430       -5     
- Misses       5212     5216       +4     
- Partials      890      891       +1     

Files with missing lines	Coverage Δ
runtime/docker/auth.go	83.92% <100.00%> (ø)

... and 1 file with indirect coverage changes