Add Dockerfiles to the templates (paritytech#4637)

As requested
[here](paritytech#3155 (comment)).

The Dockerfiles are inspired by [this
one](https://github.com/paritytech/polkadot-sdk/blob/aa32faaebf64426becb2feeede347740eb7a3908/docker/dockerfiles/polkadot/polkadot_builder.Dockerfile).

templates/minimal/.dockerignore

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+target/
+Dockerfile
+.dockerignore

templates/minimal/Dockerfile

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+FROM docker.io/paritytech/ci-unified:latest as builder
+
+WORKDIR /polkadot
+COPY . /polkadot
+
+RUN cargo fetch
+RUN cargo build --locked --release
+
+FROM docker.io/parity/base-bin:latest
+
+COPY --from=builder /polkadot/target/release/minimal-template-node /usr/local/bin
+
+USER root
+RUN useradd -m -u 1001 -U -s /bin/sh -d /polkadot polkadot && \
+	mkdir -p /data /polkadot/.local/share && \
+	chown -R polkadot:polkadot /data && \
+	ln -s /data /polkadot/.local/share/polkadot && \
+# unclutter and minimize the attack surface
+	rm -rf /usr/bin /usr/sbin && \
+# check if executable works in this container
+	/usr/local/bin/minimal-template-node --version
+
+USER polkadot
+
+EXPOSE 30333 9933 9944 9615
+VOLUME ["/data"]
+
+ENTRYPOINT ["/usr/local/bin/minimal-template-node"]

templates/parachain/.dockerignore

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+target/
+Dockerfile
+.dockerignore

templates/parachain/Dockerfile

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+FROM docker.io/paritytech/ci-unified:latest as builder
+
+WORKDIR /polkadot
+COPY . /polkadot
+
+RUN cargo fetch
+RUN cargo build --locked --release
+
+FROM docker.io/parity/base-bin:latest
+
+COPY --from=builder /polkadot/target/release/parachain-template-node /usr/local/bin
+
+USER root
+RUN useradd -m -u 1001 -U -s /bin/sh -d /polkadot polkadot && \
+	mkdir -p /data /polkadot/.local/share && \
+	chown -R polkadot:polkadot /data && \
+	ln -s /data /polkadot/.local/share/polkadot && \
+# unclutter and minimize the attack surface
+	rm -rf /usr/bin /usr/sbin && \
+# check if executable works in this container
+	/usr/local/bin/parachain-template-node --version
+
+USER polkadot
+
+EXPOSE 30333 9933 9944 9615
+VOLUME ["/data"]
+
+ENTRYPOINT ["/usr/local/bin/parachain-template-node"]

templates/solochain/.dockerignore

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+target/
+Dockerfile
+.dockerignore

templates/solochain/Dockerfile

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+FROM docker.io/paritytech/ci-unified:latest as builder
+
+WORKDIR /polkadot
+COPY . /polkadot
+
+RUN cargo fetch
+RUN cargo build --locked --release
+
+FROM docker.io/parity/base-bin:latest
+
+COPY --from=builder /polkadot/target/release/solochain-template-node /usr/local/bin
+
+USER root
+RUN useradd -m -u 1001 -U -s /bin/sh -d /polkadot polkadot && \
+	mkdir -p /data /polkadot/.local/share && \
+	chown -R polkadot:polkadot /data && \
+	ln -s /data /polkadot/.local/share/polkadot && \
+# unclutter and minimize the attack surface
+	rm -rf /usr/bin /usr/sbin && \
+# check if executable works in this container
+	/usr/local/bin/solochain-template-node --version
+
+USER polkadot
+
+EXPOSE 30333 9933 9944 9615
+VOLUME ["/data"]
+
+ENTRYPOINT ["/usr/local/bin/solochain-template-node"]