We report the Ni-doping effect on magnetism and superconductivity (SC) in an Eu-containing 112-type system $\mathrm{Eu}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}){\mathrm{As}}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.15$) by the measurements of resistivity, magnetization, and specific heat. The undoped ${\mathrm{EuFeAs}}_{2}$ undergoes a spin-density-wave (SDW) transition at ${T}_{\mathrm{SDW}}\ensuremath{\sim}105\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the Fe sublattice and a magnetic ordering at ${T}_{\mathrm{m}}\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the Eu sublattice. Complex Eu-spin magnetism is manifested by a spin-glass reentrance at ${T}_{\mathrm{SG}}\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and an additional spin reorientation at ${T}_{\mathrm{SR}}\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. With Ni doping, the SDW order is rapidly suppressed, and SC emerges in the Ni-doping range of 0.01 $\ensuremath{\le}x\ensuremath{\le}$ 0.1 where a maximum of the superconducting transition temperature ${T}_{\mathrm{c}}^{\mathrm{max}}=17.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ shows up at $x=0.04$. On the other hand, ${T}_{\mathrm{m}}$ decreases very slowly, yet ${T}_{\mathrm{SG}}$ and ${T}_{\mathrm{SR}}$ hardly change with the Ni doping. The phase diagram has been established, which suggests a very weak coupling between SC and Eu spins. The complex Eu-spin magnetism is discussed in terms of the Ruderman-Kittel-Kasuya-Yosida interactions mediated by the conduction electrons from both layers of FeAs and As surrounding ${\mathrm{Eu}}^{2+}$ ions.