chore: add docker builds

[dj8yfo]

testing was performed on this offshoot_testing_reproducible_testnet branch,

which differs from the branch of pr with this commit (relevant changes).

offshoot_testing_reproducible_testnet branch added package.metadata.near.reproducible_build
for 2 more contracts : devhub-community and devhub-discussions,
which was only needed for testing to confirm identical wasms were obtained
by running cargo near build procedure. these additional metadata sections
aren't needed in current pr, as these contracts aren't supposed to be built separately
(at least for deployments).

# confirmation, that wasms were obtained in a reproducible manner
# (nep330 metadata looks ok too)
a6144b70dd7692139b0e2b4e7be7bbb3a13aeda4966a6c6ce9277b8bb29afb61  community.devhub-test-3.testnet.wasm
5a50aecb76bb6d768791376e5ed9733dab28d1781e4e7827968d7c20f78579b1  devhub-test-3.testnet.wasm
be1a72acb6dfb54371bdeb8a3b1f81c6ab03b6b5f7649aa66d568765fef3eed3  discussions.gothamuniquehandle3.community.devhub-test-3.testnet.wasm
35d7ddea329134cad70e367c4f27e78e6539fbe145e4e669ff2fd72137ceb506  gothamuniquehandle3.community.devhub-test-3.testnet.wasm
35d7ddea329134cad70e367c4f27e78e6539fbe145e4e669ff2fd72137ceb506  standalone-devhub-community-3.testnet.wasm
be1a72acb6dfb54371bdeb8a3b1f81c6ab03b6b5f7649aa66d568765fef3eed3  standalone-devhub-discussions-3.testnet.wasm

[dj8yfo]

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