Remove custom Dockerfile and example app

[mpetrun5]

Currently, the example app is not needed anymore (the production app and example app are mostly the same) and it would be better to use the same for example app and production app as we use example for e2e tests and it would be a better test.

Implementation details

Testing details

Acceptance Criteria

[itsbobbyzzz168]

@mpetrun5 Mak sent this to you to check whether still applicable - or done as part of Sygma X

[mpetrun5]

This still applies.
Though not a big priority will be implemented when we tackle integrating sygma-core into sygma-relayer

[MakMuftic]

I will move it then to Cycle 3

[itsbobbyzzz168]

@MakMuftic will check this with @mpetrun5 - moved into cycle 1