Support ubi8 docker images

[v1v]

What does this PR do?

Add --ubi8 option to use ubi8 docker images. Verify --oss and --ubi8 options are not enabled at the same time.

Why is it important?

Support the ITs with the ubi8 minimal docker images locally and also in the CI as part of the compatibility matrix.

Related issues

Closes #900

Tests

• locally

$ BUILD_OPTS="--ubi8" ELASTIC_STACK_VERSION="8.0.0" .ci/scripts/go.sh
$ grep 'ubi8' docker-compose.yml 
      "image": "docker.elastic.co/apm/apm-server-ubi8:8.0.0-SNAPSHOT",
      "image": "docker.elastic.co/elasticsearch/elasticsearch-ubi8:8.0.0-SNAPSHOT",
$ docker ps
CONTAINER ID        IMAGE                                                               COMMAND                  CREATED             STATUS                            PORTS                                                                                                        NAMES
f4378529d269        docker.elastic.co/apm/apm-server-ubi8:8.0.0-SNAPSHOT                "/usr/local/bin/dock…"   9 seconds ago       Up 8 seconds (health: starting)   127.0.0.1:6060->6060/tcp, 127.0.0.1:8200->8200/tcp, 127.0.0.1:14250->14250/tcp, 127.0.0.1:14268->14268/tcp   localtesting_8.0.0_apm-server
a78a0dbc8094        docker.elastic.co/elasticsearch/elasticsearch-ubi8:8.0.0-SNAPSHOT   "/tini -- /usr/local…"   40 seconds ago      Up 39 seconds (healthy)           127.0.0.1:9200->9200/tcp, 9300/tcp                                                                           localtesting_8.0.0_elasticsearch

• in the CI i forced the build see here

[axw]

LGTM, thank you! Just one small comment about the binary name and:

• I think Logstash is missing?
• Could we please add UBI8 to the integration testing matrix?

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Started by timer]

• Start Time: 2020-09-22T03:02:00.761+0000

• Duration: 38 min 41 sec

Test stats 🧪

Test	Results
Failed	0
Passed	416
Skipped	12
Total	428

[axw]

Perfect, thank you!