[chore/docs] Add /gotosocial/.cache to Docker container, document GTS_WAZERO_COMPILATION_CACHE

[tsmethurst]

Description

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This pull request adds the optional /gotosocial/.cache volume in the Dockerfile, and documents usage of GTS_WAZERO_COMPILATION_CACHE to set the compilation cache dir, for both Docker and binary deployments (with systemd).

Closes #3233

[NyaaaWhatsUpDoc]

i think it would be better to further specify what this cache is for in this directory, in case we ever have further need for using /gotosocial/.cache.

so making GTS_WAZERO_COMPILATION_CACHE=/gotosocial/.cache/wazero

[tsmethurst]

Mm, well wazero creates directories of its own inside that cache directory like wazero_vx.x.x or whatever, so I don't think it's really necessary, then it would end up being ~/gotosocial/.cache/wazero/wazero_vx.x.x