Zn is a metal which forms cations 

But why Zn container acts as an anode in a dry cell

In the dry cell, the Zn container acts as an anode and graphite is act as a cathode surrounded by MnO₂ + Carbon.

here is a paste of NH₄Cl and ZnCl₂ is filled in between the electrodes.

Cell reaction: 

At anode:

Zn_(s) → Zn²⁺ + 2e^- 

At cathode:

2MnO_2(s)  + NH₄⁺_(aq) + 2e^- →Mn₂O_3(s) + 2NH_3(g) + H₂O

 

Zn²⁺ combines with NH₃ to form diammonium Zn(II) cation.

The dry cell has a short life, as NH₄Cl corrodes Zn container even when the cell is not in use.