Move to official Rust Alpine Docker image

The previous image appears to be mostly unmaintained:
emk/rust-musl-builder#147

Author: shepmaster

.github/workflows/ci.yml

@@ -194,17 +194,17 @@ jobs:
         args: "--manifest-path top-crates/Cargo.toml --check"
     - name: Build backend
       run: |-
-        mkdir -p ui/target; docker run --rm -v $PWD/ui:/ui -v ~/.cargo/git:/home/rust/.cargo/git -v ~/.cargo/registry:/home/rust/.cargo/registry --workdir /ui ekidd/rust-musl-builder:stable bash -c $'
-          sudo chown -R rust:rust /home/rust/.cargo /ui/target;
+        mkdir -p ui/target; docker run --rm -v $PWD/ui:/ui -v ~/.cargo/git:/root/.cargo/git -v ~/.cargo/registry:/root/.cargo/registry --workdir /ui rust:alpine bash -c $'
+          apk add musl-dev openssl-dev
 
           # Unit tests
-          cargo test --locked --target=x86_64-unknown-linux-musl --no-run;
-          test_bin=$(find target/x86_64-unknown-linux-musl/debug/deps/ -type f -perm -a=x);
+          cargo test --locked --no-run;
+          test_bin=$(find target/debug/deps/ -type f -perm -a=x);
           mv "${test_bin}" target/unit_tests;
 
           # Primary binary
-          cargo build --locked --target=x86_64-unknown-linux-musl --release;
-          mv target/x86_64-unknown-linux-musl/release/ui target/ui;
+          cargo build --locked --release;
+          mv target/release/ui target/ui;
         '
     - name: Restore permissions
       run: sudo chown -R runner:docker ~/.cargo/ ui/target

ci/workflows.yml

@@ -298,21 +298,21 @@ workflows:
                 run
                 --rm
                 -v $PWD/ui:/ui
-                -v ~/.cargo/git:/home/rust/.cargo/git
-                -v ~/.cargo/registry:/home/rust/.cargo/registry
+                -v ~/.cargo/git:/root/.cargo/git
+                -v ~/.cargo/registry:/root/.cargo/registry
                 --workdir /ui
-                ekidd/rust-musl-builder:stable
+                rust:alpine
                 bash -c $'
-                  sudo chown -R rust:rust /home/rust/.cargo /ui/target;
+                  apk add musl-dev openssl-dev
 
                   # Unit tests
-                  cargo test --locked --target=x86_64-unknown-linux-musl --no-run;
-                  test_bin=$(find target/x86_64-unknown-linux-musl/debug/deps/ -type f -perm -a=x);
+                  cargo test --locked --no-run;
+                  test_bin=$(find target/debug/deps/ -type f -perm -a=x);
                   mv "${test_bin}" target/unit_tests;
 
                   # Primary binary
-                  cargo build --locked --target=x86_64-unknown-linux-musl --release;
-                  mv target/x86_64-unknown-linux-musl/release/ui target/ui;
+                  cargo build --locked --release;
+                  mv target/release/ui target/ui;
                 '
 
           - name: "Restore permissions"