Add yarn install to codespace dockerfile

Author: metaproph3t

.devcontainer/Dockerfile

@@ -47,7 +47,11 @@ RUN sh -c "$(curl -sSfL https://release.solana.com/v${SOLANA_CLI}/install)"
 RUN cargo install --git https://github.com/coral-xyz/anchor avm --locked --force
 RUN avm install ${ANCHOR_CLI} && avm use ${ANCHOR_CLI}
 
+# set up keypair
 RUN solana-keygen new --no-bip39-passphrase
 
+# install dependencies
+RUN yarn
+
 WORKDIR /workdir
 #be sure to add `/root/.avm/bin` to your PATH to be able to run the installed binaries