RFR: 8265690: Use the latest Ubuntu base image version in Docker testing [v2]

[Aleksey Shipilev]

> Use latest Ubuntu base 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
>  - [ ] `hotspot_containers` tests on Debian 9 (glibc 2.24) -- continues to pass
>  - [ ] `hotspot_containers` tests on Centos 7 (glibc 2.17) -- continues to pass

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains one additional commit since the last revision:

  8265690: Use latest Ubuntu base image version in Docker testing

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3605/files
  - new: https://git.openjdk.java.net/jdk/pull/3605/files/d36717b5..ad904a7d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3605&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3605&range=00-01

  Stats: 869 lines in 84 files changed: 516 ins; 194 del; 159 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