Fix python minor version to 3.7

jstriebel · web-flow · commit 17097b9f4ad1 · 2019-01-10T15:31:22.000+01:00
`3.6` is also fine, but we should fix it for now, I guess.
diff --git a/Dockerfile b/Dockerfile
@@ -1,4 +1,4 @@
-FROM python:3
+FROM python:3.7
 
 ENV LZ4_VERSION=1.8.1.2
 
