Add exec to replace shell inside entrypoint with the actual binary

[voron]

We need to pass signals to the actual binary to handle it correctly. The easiest way to accomplish this is to replace entrypoint's shell with the binary instead of telling the shell to execute this binary as a child process.
Temporary workaround for k8s with exec works fine in my tests

command:
  - /bin/bash
args:
  - -c
  - sqlx database setup && exec zksync_external_node

During k8s pod termination I get following lines in logs

INFO zksync_external_node: Stop signal received, shutting down
...
INFO zksync_core::api_server::healthcheck: Healthcheck server shut down

[hatemosphere]

we're doing the same thing internally, so LGTM i guess

[RomanBrodetski]

@voron please rebase your branch off main

[voron]

@voron please rebase your branch off main

@RomanBrodetski will merge from GitHub's sync fork suit your needs ?

[voron]

I see it's been handled by #134