revert entrypoint -> cmd + clean some alpine stuff

Dockerfile

@@ -2,6 +2,7 @@ FROM "alpine:3.7"
 LABEL maintainer="Cabillot Julien <dockerimages@cabillot.eu>"
 
 RUN apk add --no-cache mysql-client && \
+    rm -rf "/usr/share/apk/keys" && \
     adduser -D muser
 
 # Add Tini
@@ -12,4 +13,4 @@ RUN apk add --no-cache mysql-client && \
 
 USER "muser"
 
-CMD [ "/usr/bin/mysql" ]
+ENTRYPOINT [ "/usr/bin/mysql" ]