dockerfile: copy the api dir before go mod download #1824

Job	Run time
Build image	2m 37s
drenv tests (3.9)	1m 37s
Golangci Lint	19s
Linters	56s
drenv tests (3.10)	1m 34s
ramenctl tests (3.9)	14s
drenv tests (3.11)	1m 23s
ramenctl tests (3.10)	16s
ramenctl tests (3.11)	23s
Unit tests	4m 37s
Check artifacts and operator deployment (1.20.7)	2m 19s
Publish built image	1m 38s
	17m 53s

Provide feedback