Fix .devcontainer dockerfile

[rvirtaha]

npm is not defined in the dev.Dockerfile

 => ERROR [vaalikoppi dev_container_auto_added_stage_label 10/11] RUN npm  0.3s
                                                                                
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 > [vaalikoppi dev_container_auto_added_stage_label 10/11] RUN npm install && npx playwright install --with-deps:
0.262 /bin/sh: 1: npm: not found
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failed to solve: process "/bin/sh -c npm install && npx playwright install --with-deps" did not complete successfully: exit code: 127