Add BLOCKSCOUT_HOST to docker setup

[vbaranov]

Motivation

Add BLOCKSCOUT_HOST to docker setup

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 5fdbe49c-cccf-42e9-86ac-03fcfee81a7e

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.234%

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Totals
Change from base Build 1874fb86-4453-454a-8201-e072d98a3cb4:	0.0%
Covered Lines:	5377
Relevant Lines:	7147

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💛 - Coveralls