Scramble a Megaminx! Scramble a Megaminx!
A megaminx is a dodecahedral version of the familiar Rubik's cube. Since 2008, the World Cube Association uses Stefan Pochmann's megaminx scrambler. The scrambles look like this:
R-- D++ R++ D-- R++ D-- R++ D++ R-- D-- U'
R++ D-- R++ D-- R-- D++ R-- D-- R-- D-- U'
R++ D++ R-- D++ R++ D++ R++ D++ R-- D++ U 
R-- D-- R++ D++ R++ D++ R-- D++ R++ D++ U 
R-- D++ R++ D-- R++ D-- R++ D++ R++ D++ U 
R++ D++ R++ D++ R-- D-- R-- D-- R-- D-- U'
R-- D++ R-- D++ R++ D++ R++ D++ R++ D++ U 

A scramble has seven lines. Each line has 10 R and D moves, starting with R and alternating R and D moves. Following each letter is randomly chosen direction ++ or -- with equal probability. At the end of the line is randomly chosen with equal probability U (trailing space form U  allowed) or U'. A trailing newline at the end of the output is permitted.
Your code must take no input and must produce a random scramble.
Ruby, 73 71 bytes
Saved two bytes thanks to @vrintle (one directly and another with further golfing)

77.times{|i|$&gt;&lt;&lt;(~i%11&gt;0?:RD[i%2]+'+-'[rand 2]*2+' ':?U+?'*rand(2)+$/)}

Try it online!
$/ is the input record separator, which is a newline by default.

In Ruby 2.7 (newer than the version on TIO), we get 70 bytes using numbered block parameters:
77.times{$&gt;&lt;&lt;(~_1%11&gt;0?:RD[_1%2]+'+-'[rand 2]*2+' ':?U+?'*rand(2)+$/)}