Fix backend Dockerfile

[akira]

See discussion here: #109

Issue is:

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 > [backend 5/6] RUN rm poetry.lock:
0.125 rm: cannot remove 'poetry.lock': No such file or directory
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failed to solve: process "/bin/sh -c rm poetry.lock" did not complete successfully: exit code: 1

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[nfcampos]

Superseded by #120