chore(deps): update maven docker tag to v3.8

This MR contains the following updates:

Package	Type	Update	Change
maven	stage	minor	3.6-openjdk-11 -> 3.8-openjdk-11

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Dockerfile

@@ -4,7 +4,7 @@
 
 
 # the first stage of our build will use a maven 3.6 parent image
-FROM maven:3.6-openjdk-11 AS MAVEN_BUILD
+FROM maven:3.8-openjdk-11 AS MAVEN_BUILD
 
 # We need VIPSCommon too, so the source code must be available in the parent folder
 COPY ./VIPSCommon ./VIPSCommon