Convert backend to use alpine to get libssl3

[mipyykko]

Debian buster (nor bullseye) have libssl3, so a mismatch between Node.js 18, Prisma and the distribution libssl version here. Alpine has it, though, and migrating the Dockerfile to use that was easy.

Also, Prisma connection url no longer accepts quoted strings.

[codecov]

Codecov Report

Merging #1198 (9c36878) into master (daa33e3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1198   +/-   ##
=======================================
  Coverage   64.30%   64.30%           
=======================================
  Files         125      125           
  Lines        5306     5306           
  Branches     1238     1238           
=======================================
  Hits         3412     3412           
  Misses       1767     1767           
  Partials      127      127           

Flag	Coverage Δ
backend	64.30% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
backend/graphql/Completion/index.ts	100.00% <ø> (ø)
backend/graphql/Course/index.ts	100.00% <ø> (ø)
backend/graphql/StudyModule/index.ts	100.00% <ø> (ø)
backend/graphql/User/index.ts	100.00% <ø> (ø)
backend/graphql/index.ts	100.00% <ø> (ø)

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