RFR: 8265690: Use latest base image version in Docker testing

[Aleksey Shipilev]

Use latest image to get latest glibc, so that host-built JDK has a higher chance to pass. Current "oraclelinux:7.6" image apparently has glibc 2.27. Please see the bug for more discussion.

Additional testing:
 - [x] `hotspot_containers` tests on Ubuntu 20.04 (glibc 2.31) -- used to fail, now passes
 - [x] `hotspot_containers` tests on Debian 9 (glibc 2.24) -- continues to pass
 - [x] `hotspot_containers` tests on Centos 7 (glibc 2.17) -- continues to pass

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Commit messages:
 - 8265690: Use latest base image version in Docker testing

Changes: https://git.openjdk.java.net/jdk/pull/3605/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3605&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265690
  Stats: 8 lines in 1 file changed: 0 ins; 7 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3605.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3605/head:pull/3605

PR: https://git.openjdk.java.net/jdk/pull/3605