chore(stacks): Add Docker QA Stack for 7.21.0-SNAPSHOT

[psavidis]

Related-to: camunda/camunda-bpm-platform/issues/3339

[tmetzke]

Thanks, @psavidis! Two things to consider:

• Did you stick to https://github.com/camunda/portainer-templates#how-to-create-a-new-stack-type-template? I cannot see the adjustments in all the files mentioned there.
• With 7.20, we drop support for WAS9 (see HERE). I think we can skip creating a new stack for this.

[psavidis]

Thanks, @psavidis! Two things to consider:

• Did you stick to https://github.com/camunda/portainer-templates#how-to-create-a-new-stack-type-template? I cannot see the adjustments in all the files mentioned there.
• With 7.20, we drop support for WAS9 (see HERE). I think we can skip creating a new stack for this.

• Hey, thanks for reminding me of the guide. I've reverted everything and started from scratch with a clean setup (i was missing the python command).
• Also, didn't include WAS 9 as recommended.

[tmetzke]

👍 The addition itself looks good.
🔧 There are some minor things to change here that make this more consistent.

[tmetzke]

🔧 I think you can remove the new tomcat stacks, we won't need them.
👍 Other than that, we're good to go I think