It is normal for the older, lower leaves on cucumber vines to yellow and dry up, however, the yellowing of younger leaves can be the result of a nutrient imbalance.  Although the Epsom salts is not necessarily harmful to the plant, the soil most likely contains adequate magnesium and you should concentrate on the NPK applications.  Don't overdo the fertilization, since there can also be a build-up of salts in the soil and this would then have a detrimental effect on the plant.   Also, make sure the container is draining adequately.  Excess water in the soil will also cause chlorosis.
LS