fix(sem): Use PR number for docker image (#340)

## 📝 Description

We are moving from executing CI on every branch to executing on PR's. 
## ✅ Checklist
- [ ] I have tested this change
- [ ] This change requires documentation update

Author: VeljkoMaksimovic

Makefile

@@ -7,6 +7,8 @@ TAG_NAME=$(shell git describe --exact-match --tags HEAD 2>/dev/null)
 #
 ifneq ($(TAG_NAME),)
 	export BRANCH?=$(shell git branch --contains tags/$(TAG_NAME) | head -n 1 | sed '/HEAD/d' | sed 's/[^a-z]//g' | cut -c 1-40)
+else ifneq ($(SEMAPHORE_GIT_PR_NUMBER),)
+	export BRANCH?=pr$(SEMAPHORE_GIT_PR_NUMBER)
 else
 	export BRANCH?=$(shell git rev-parse --abbrev-ref HEAD | sed 's/[^a-z]//g' | cut -c 1-40)
 endif