Switch to a slimmer distroless base image.

At a high level, it switches us to a distroless base container image, but that also includes several related bits:

- Add a writable /tmp but make the rest of our filesystems read-only at runtime.

- Condense our main server binaries into a single pinniped-server binary. This saves a bunch of space in
  the image due to duplicated library code. The correct behavior is dispatched based on `os.Args[0]`, and
  the `pinniped-server` binary is symlinked to `pinniped-concierge` and `pinniped-supervisor`.

- Strip debug symbols from our binaries. These aren't really useful in a distroless image anyway and all the
  normal stuff you'd expect to work, such as stack traces, still does.

- Add a separate `pinniped-concierge-kube-cert-agent` binary with "sleep" and "print" functionality instead of
  using builtin /bin/sleep and /bin/cat for the kube-cert-agent. This is split from the main server binary
  because the loading/init time of the main server binary was too large for the tiny resource footprint we
  established in our kube-cert-agent PodSpec. Using a separate binary eliminates this issue and the extra
  binary adds only around 1.5MiB of image size.

- Switch the kube-cert-agent code to use a JSON `{"tls.crt": "<b64 cert>", "tls.key": "<b64 key>"}` format.
  This is more robust to unexpected input formatting than the old code, which simply concatenated the files
  with some extra newlines and split on whitespace.

- Update integration tests that made now-invalid assumptions about the `pinniped-server` image.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

Dockerfile

@@ -16,26 +16,18 @@ RUN \
   --mount=type=cache,target=/cache/gocache \
   --mount=type=cache,target=/cache/gomodcache \
   mkdir out && \
-  GOCACHE=/cache/gocache \
-  GOMODCACHE=/cache/gomodcache \
-  CGO_ENABLED=0 \
-  GOOS=linux \
-  GOARCH=amd64 \
-  go build -v -ldflags "$(hack/get-ldflags.sh)" -o out \
-    ./cmd/pinniped-concierge/... \
-    ./cmd/pinniped-supervisor/... \
-    ./cmd/local-user-authenticator/...
+  export GOCACHE=/cache/gocache GOMODCACHE=/cache/gomodcache CGO_ENABLED=0 GOOS=linux GOARCH=amd64 && \
+  go build -v -ldflags "$(hack/get-ldflags.sh) -w -s" -o /usr/local/bin/pinniped-concierge-kube-cert-agent ./cmd/pinniped-concierge-kube-cert-agent/main.go && \
+  go build -v -ldflags "$(hack/get-ldflags.sh) -w -s" -o /usr/local/bin/pinniped-server ./cmd/pinniped-server/main.go && \
+  ln -s /usr/local/bin/pinniped-server /usr/local/bin/pinniped-concierge && \
+  ln -s /usr/local/bin/pinniped-server /usr/local/bin/pinniped-supervisor && \
+  ln -s /usr/local/bin/pinniped-server /usr/local/bin/local-user-authenticator
 
-# Use a Debian slim image to grab a reasonable default CA bundle.
-FROM debian:10.10-slim AS get-ca-bundle-env
-RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates && rm -rf /var/lib/apt/lists/* /var/cache/debconf/*
+# Use a distroless runtime image with CA certificates, timezone data, and not much else.
+FROM gcr.io/distroless/static:nonroot@sha256:c9f9b040044cc23e1088772814532d90adadfa1b86dcba17d07cb567db18dc4e
 
-# Use a runtime image based on Debian slim.
-FROM debian:10.10-slim
-COPY --from=get-ca-bundle-env /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
-
-# Copy the binaries from the build-env stage.
-COPY --from=build-env /work/out/ /usr/local/bin/
+# Copy the server binary from the build-env stage.
+COPY --from=build-env /usr/local/bin /usr/local/bin
 
 # Document the ports
 EXPOSE 8080 8443
@@ -44,4 +36,4 @@ EXPOSE 8080 8443
 USER 1001:1001
 
 # Set the entrypoint
-ENTRYPOINT ["/usr/local/bin/pinniped-concierge"]
+ENTRYPOINT ["/usr/local/bin/pinniped-server"]