feat(docker): Allow dotcom-1 and dotcom-2 start in parallel in docker compose

[joshlarson]

The current setup has dotcom-2 wait for dotcom-1 to finish starting up before it even starts trying. This PR speeds up docker compose startup time, since it doesn't force dotcom-2 to wait anymore.

Before

flowchart LR
    dotcom-1 --> dotcom-2 --> nginx

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After

flowchart LR
    dotcom-1 --> nginx
    dotcom-2 --> nginx

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Historical context

• Looks like the chaining was added in Chain service launch to reduce conflicts #1951 in order to keep the parallel elixir ... mix phx.server processes from stepping on each other. That's because they were both (under the hood) running mix deps.compile and writing to the same _build directory, which was mounted in by docker-compose.
• But with the changes introduced in fix: Don't overwrite dep directories in docker-compose'ed containers #2182, dotcom-1 and dotcom-2 aren't sharing the same _build directory anymore, and mix deps.compile is run as part of building the image, which means it doesn't happen again when the containers are starting up, so chaining is no longer necessary.