Revert "HADOOP-16054. Update Dockerfile to use Bionic."

[aajisaka]

This reverts commit 367833c.

The precommit job fails to add comments to JIRA and GitHub after the upgrade of the build image. Revert the upgrade to see if the precommit job can add comment to GitHub.

[hadoop-yetus]

(!) A patch to the testing environment has been detected.
Re-executing against the patched versions to perform further tests.
The console is at https://builds.apache.org/job/hadoop-multibranch/job/PR-1911/1/console in case of problems.

[aajisaka]

It's 10:40 pm in Japan and I will not be available after a few hours. Feel free to merge this if hadoop-yetus can add comment to this jira.

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	29m 20s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+0 🆗	shelldocs	0m 0s	Shelldocs was not available.
+1 💚	@author	0m 0s	The patch does not contain any @author tags.
		_ trunk Compile Tests _
+0 🆗	mvndep	1m 14s	Maven dependency ordering for branch
+1 💚	shadedclient	15m 43s	branch has no errors when building and testing our client artifacts.
		_ Patch Compile Tests _
+0 🆗	mvndep	0m 29s	Maven dependency ordering for patch
+1 💚	hadolint	0m 2s	There were no new hadolint issues.
+1 💚	shellcheck	0m 0s	There were no new shellcheck issues.
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	shadedclient	16m 8s	patch has no errors when building and testing our client artifacts.
		_ Other Tests _
+1 💚	asflicense	0m 30s	The patch does not generate ASF License warnings.
		65m 11s

Subsystem	Report/Notes
Docker	Client=19.03.8 Server=19.03.8 base: https://builds.apache.org/job/hadoop-multibranch/job/PR-1911/1/artifact/out/Dockerfile
GITHUB PR	#1911
Optional Tests	dupname asflicense hadolint shellcheck shelldocs
uname	Linux e61eb54bba52 4.15.0-74-generic #84-Ubuntu SMP Thu Dec 19 08:06:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/hadoop.sh
git revision	trunk / 28afdce
Max. process+thread count	308 (vs. ulimit of 5500)
modules	C: U:
Console output	https://builds.apache.org/job/hadoop-multibranch/job/PR-1911/1/console
versions	git=2.7.4 maven=3.3.9 shellcheck=0.3.7 hadolint=1.11.1-0-g0e692dd
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[tasanuma]

@aajisaka Thanks for the PR.
hadoop-yetus commented. +1. Will merge it.

[aajisaka]

Thanks @tasanuma for reviewing and merging this