alpine:3.4 -> alpine:3 (3.4 is not supported in newer platforms) (#146)

Author: idanmiara

tests/resources/invalid_parse_dockerfile/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.4
+FROM alpine:3
 
 RUN apk update
 # Parse error - ARG statment without idnetifier

tests/resources/invalid_run_dockerfile/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.4
+FROM alpine:3
 
 RUN apk update
 # Run error - file doesn't exists

tests/test_image_build.py

@@ -79,7 +79,7 @@ async def build():
         await build()
 
     captured = capsys.readouterr()
-    assert "Step 1/3 : FROM alpine:3.4" in captured.err
+    assert "Step 1/3 : FROM alpine:3" in captured.err
     assert "Step 2/3 : RUN apk update" in captured.err
     assert "Step 3/3 : RUN file_not_exists.sh" in captured.err
     assert "file_not_exists.sh: not found" in captured.err