Remove explicit /srv/ usage from docker-compose files

[paskal]

We have plenty of paths used in the application (like AVATAR_FS_PATH, IMAGE_FS_PATH), but two of them are hardcoded in examples all over the code for historical reasons.

I found that by default in Docker, the path would anyway resolve to the value we are setting it explicitly to, so it doesn't make sense to set a few variables we are setting now explicitly.

I was confused by these two variables when figuring out how to run the application outside of docker by looking at the default variables we propose in docker-compose.yml.

[coveralls]

Pull Request Test Coverage Report for Build 1803644200

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 84.563%

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Totals
Change from base Build 1803346585:	0.03%
Covered Lines:	6075
Relevant Lines:	7184

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💛 - Coveralls

[umputun]

just to make sure: the reason to remove it is /srvWORKDIR in docker plus default values in cmd/server.go producing the same path anyway, correct?

[paskal]

Correct, the binary starts from /srv and works with default values starting with ./var/, resulting in /srv/var/ usage.