build: Correctly set each target

Dockerfile

@@ -1,18 +1,44 @@
 FROM rust:1.80 AS builder
+ARG TARGETPLATFORM
 WORKDIR /usr/src/
-RUN rustup target add x86_64-unknown-linux-musl
+
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then  \
+    rustup target add x86_64-unknown-linux-musl;  \
+    elif [ "$TARGETPLATFORM" = "linux/arm/v7" ]; then  \
+    rustup target add armv7-unknown-linux-musleabihf;  \
+    elif [ "$TARGETPLATFORM" = "linux/arm64" ]; then  \
+    rustup target add aarch64-unknown-linux-musl;  \
+    else exit 1;  \
+    fi
 
 RUN USER=root cargo new whatismyip
 WORKDIR /usr/src/whatismyip
 COPY Cargo.toml Cargo.lock ./
-RUN cargo build --release
+
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ] ; then  \
+      cargo build --release --target=x86_64-unknown-linux-musl ;  \
+    elif [ "$TARGETPLATFORM" = "linux/arm/v7" ] ; then  \
+      cargo build --release --target=armv7-unknown-linux-musleabihf ;  \
+    elif [ "$TARGETPLATFORM" = "linux/arm64" ] ; then  \
+      cargo build --release --target=aarch64-unknown-linux-musl ;  \
+    else exit 1 ;  \
+    fi
+
 
 COPY src ./src
-RUN cargo install --target x86_64-unknown-linux-musl --path .
+
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then  \
+    cargo install --target=x86_64-unknown-linux-musl --path . ;  \
+    elif [ "$TARGETPLATFORM" = "linux/arm/v7" ]; then  \
+    cargo install --target=armv7-unknown-linux-musleabihf --path . ;  \
+    elif [ "$TARGETPLATFORM" = "linux/arm64" ]; then  \
+    cargo install --target=aarch64-unknown-linux-musl --path . ;  \
+    else exit 1;  \
+    fi
 
 # Bundle Stage
 FROM scratch
 COPY --from=builder /usr/local/cargo/bin/whatismyip .
 RUN ["./whatismyip", "-l"]
 USER 1000
-ENTRYPOINT ["./whatismyip"]
+ENTRYPOINT ["./whatismyip"]