Add Dockerfile

This allows using xk6 via Docker, without having to setup a local Go
environment or install xk6.

Unfortunately, file permissions with mounted volumes are still an
issue[1], without a simple, native fix. fixuid[2] seems to be the modern
solution to this, and works well from my tests.

Part of #8

[1]: moby/moby#7198

[2]: https://github.com/boxboat/fixuid

Dockerfile

@@ -0,0 +1,35 @@
+ARG GO_VERSION=1.20.1
+ARG VARIANT=bullseye
+FROM golang:${GO_VERSION}-${VARIANT} as builder
+
+WORKDIR /build
+
+COPY . .
+
+ARG GOFLAGS="-ldflags=-w -ldflags=-s"
+RUN CGO_ENABLED=0 go build -o xk6 -trimpath ./cmd/xk6/main.go
+
+
+FROM golang:${GO_VERSION}-${VARIANT}
+
+RUN addgroup --gid 1000 xk6 && \
+    adduser --uid 1000 --ingroup xk6 --home /home/xk6 --shell /bin/sh --disabled-password --gecos "" xk6
+
+ARG FIXUID_VERSION=0.5.1
+RUN USER=xk6 && \
+    GROUP=xk6 && \
+    curl -fSsL https://github.com/boxboat/fixuid/releases/download/v${FIXUID_VERSION}/fixuid-${FIXUID_VERSION}-linux-amd64.tar.gz | tar -C /usr/local/bin -xzf - && \
+    chown root:root /usr/local/bin/fixuid && \
+    chmod 4755 /usr/local/bin/fixuid && \
+    mkdir -p /etc/fixuid && \
+    printf "user: $USER\ngroup: $GROUP\n" > /etc/fixuid/config.yml
+
+COPY --from=builder /build/xk6 /usr/local/bin/
+
+COPY docker-entrypoint.sh /usr/local/bin/entrypoint.sh
+
+WORKDIR /xk6
+RUN chown xk6:xk6 /xk6
+USER xk6
+
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]

docker-entrypoint.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+set -e
+
+eval "$(fixuid)"
+
+exec /usr/local/bin/xk6 "$@"