RFR(T): 8220313: [TESTBUG] Update base image for Docker testing to OL 7.6

[mikhailo.seledtsov at oracle.com]

Please review this trivial change:

--- a/test/hotspot/jtreg/runtime/containers/docker/Dockerfile-BasicTest
+++ b/test/hotspot/jtreg/runtime/containers/docker/Dockerfile-BasicTest
@@ -1,4 +1,4 @@
-FROM oraclelinux:7.2
+FROM oraclelinux:7.6

diff --git a/test/jdk/jdk/internal/platform/docker/Dockerfile-BasicTest 
b/test/jdk/jdk/internal/platform/docker/Dockerfile-BasicTest
--- a/test/jdk/jdk/internal/platform/docker/Dockerfile-BasicTest
+++ b/test/jdk/jdk/internal/platform/docker/Dockerfile-BasicTest
@@ -1,4 +1,4 @@
-FROM oraclelinux:7.2
+FROM oraclelinux:7.6


Testing: ran both sets of tests: 
test/hotspot/jtreg/runtime/containers/docker/ AND 
test/jdk/jdk/internal/platform/docker/
All PASS


Thank you,

Misha