The generator matrix

 1  1  1  1  X
 0  X  0 X^2+X X^2
 0  0 X^2 X^2 X^2

generates a code of length 5 over Z2[X]/(X^3) who´s minimum homogenous weight is 4.

Homogenous weight enumerator: w(x)=1x^0+18x^4+32x^5+8x^6+5x^8

The gray image is a linear code over GF(2) with n=20, k=6 and d=8.
As d=8 is an upper bound for linear (20,6,2)-codes, this code is optimal over Z2[X]/(X^3) for dimension 6.
This code was found by Heurico 1.16 in 0.000116 seconds.