fix: Cross-compilation to ARM64 (#4142)

* feat(docker/docs): explain how building lemmy works

Signed-off-by: Enzo NOCERA <enzo@nocera.eu>

* feat: add arm build

* review: rename script & fix typo

* feat(ci): allow cross platform compilation

* feat(ci): prettier

* fix(docker): fix base image name

* fix: add dockerfile in CI path

Signed-off-by: Enzo Nocera <enzo@nocera.eu>

* fix(docker): fix runner name

* fix(docker): fix builder base image

* fix(docker): fix builder base image platform

* fix(docker): avoid using the wrapper adduser/addgroup

* feat: avoid adding the whole docker directory in the build context

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Signed-off-by: Enzo NOCERA <enzo@nocera.eu>
Signed-off-by: Enzo Nocera <enzo@nocera.eu>
Co-authored-by: Dessalines <dessalines@users.noreply.github.com>
Co-authored-by: Nutomic <me@nutomic.com>