Speedup building docker images in CI #91

Job	Run time
Build for Linux (linux, amd64)	1m 10s
Build for Windows (windows, amd64)	54s
Linting	1m 59s
Build for Linux (linux, 386)	1m 7s
Unit tests	2m 4s
WASM build test	0s
Initial tests passed	2s
Upgrade tests	6m 54s
Sytest (SQLite native)	4m 43s
Integration tests	4m 45s
Sytest (SQLite Cgo)	4m 46s
Complement (SQLite native)	4m 4s
Sytest (PostgreSQL)	6m 6s
Upgrade tests from HEAD-2	7m 3s
Complement (SQLite Cgo)	4m 37s
Complement (PostgreSQL)	9m 16s
Integration tests passed	4s
Update Docker images	0s
	59m 34s

Provide feedback