add small example for postgres (using pgx)

[anuragsoni]

This adds a very small example of a mirage unikernel interacting with a postgresql database using the newly published pgx_lwt_mirage library.

[MagnusS]

Restarted the CI build. It failed because of a temporary error with ocamlgraph which should be fixed now.

[anuragsoni]

Restarted the CI build. It failed because of a temporary error with ocamlgraph which should be fixed now.

Thank you! The current test failures look unrelated, but if there is anything more that i should update for this PR do let me know 😄

[hannesm]

would you mind to add device-usage/pgx to the list of BASE_TESTS in the Makefile at the root of this repository? this way, your pgx example will actually be compiled.

the CI errors (test runs on unix) are failing, which is unrelated to your PR, please ignore them.

[anuragsoni]

would you mind to add device-usage/pgx to the list of BASE_TESTS in the Makefile at the root of this repository? this way, your pgx example will actually be compiled.

I figured i was missing something for tests. Updated the Makefile. Thanks!

[anuragsoni]

I just realized that pgx is only available starting from ocaml 4.08. Is there a way to specify that it shouldn't be built on older ocaml versions that are tested in CI?

[hannesm]

CI is testing this unikernel now, and discovering that it requires OCaml 4.08.0 (thus the 4.07 job fails). Merging. In a subsequent PR, I'll bump the OCaml versions used by CI.

[anuragsoni]

Thanks!