Fix incrementation of alpha, beta and rc preversion variable in cases if there are pre already.

[DRITE]

This PR fixes issue #4

Provides next logic:

1. IF there is no $preversion, THEN preversion=0
2. ELSE IF pre != "-alpha", THEN preversion=1 (same as before)
3. ELSE increment preversion

The same for beta and rc.

Here some tests results:

bash entrypoint.sh 1.15.0-beta1 alpha
create alpha-release version: 1.15.0-beta1 -> 1.15.0-alpha1

bash entrypoint.sh 1.15.0-beta1 beta
create beta-release version: 1.15.0-beta1 -> 1.15.0-beta2

bash entrypoint.sh 1.15.0-alpha22 alpha
create alpha-release version: 1.15.0-alpha22 -> 1.15.0-alpha23

bash entrypoint.sh 1.15.0 rc
create rc-release version: 1.15.0 -> 1.15.0-rc0