Cache downloaded Go dependencies and Atlantis compilation in Docker (#2738)

This has the potential to speed up rebuilds of the Atlantis container
significantly, especially for non-x86 platforms which are emulated when
done in GitHub Actions.

It works by using cache mounts:
https://docs.docker.com/engine/reference/builder/#run---mounttypecache.
They mean we can have specific paths set that are carried over from one
image build to the next, regardless of which files are changed in the
project. The files are however at the same time not part of the final
image, which reduces the generated "builder" image from 1.66 GB to just
386 MB on the current main branch.

In my local tests, which doesn't make use of emulation, I saw a drop
for the "go build" step going from taking just over 40 seconds with an
empty cache to around 1 second when I make a one character change to a
Go source file. That is otherwise a change that prior to this would
cause a complete rebuild of the Docker image layer.

Co-authored-by: nitrocode <7775707+nitrocode@users.noreply.github.com>

Dockerfile

@@ -3,7 +3,9 @@ FROM golang:1.19.3-alpine AS builder
 
 WORKDIR /app
 COPY . /app
-RUN CGO_ENABLED=0 go build -trimpath -ldflags "-s -w" -v -o atlantis .
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    CGO_ENABLED=0 go build -trimpath -ldflags "-s -w" -v -o atlantis .
 
 # Stage 2
 # The runatlantis/atlantis-base is created by docker-base/Dockerfile.